To all the people trying to disprove the curse by saying injuries happen to players who aren't on the cover: logically, your assertion doesn't disprove the existence of the curse. All your examples prove is the fact that injuries can happen to players who aren't on the cover of Madden, which no one is disputing. You would have to show an example of a player who DIDN'T get injured or have a poor year while being on the cover of Madden.
The only player who made it relatively unscathed after being chosen for the cover of Madden was Larry Fitzgerald, who shared it with Troy Polmalu. Granted, Fitzgerald still got injured and couldn't attend the Pro Bowl after losing the Super Bowl. In any event, this needs to be reposted:
Garrison Hearst appearing on the 1999 PAL cover was known to be the "start" of the curse, as after his appearance, he broke his ankle and was out for 2 seasons, and his performance was never the same.
After Barry Sanders' appearance on the 2000 cover, Sanders abruptly retired before training camp, falling just short of the record for most career rushing yards.
Dorsey Levens appeared on the PAL version of the 2000 game, and the Green Bay Packers, whom Levens was playing for at the time, failed to make the playoffs, and Levens was eventually released the year later.
After Eddie George appeared on the 2001 cover, the Tennessee Titans lost in Divisional round in the 2001 playoffs. Late in the 4th quarter with the Titans down 17-10, a Steve McNair pass was bobbled by George and intercepted by Ray Lewis for a touchdown, sealing the game. George also never averaged more than 3.4 yards per carry for the rest of his career.
Daunte Culpepper led the Minnesota Vikings to the playoffs in 2000, but after appearing on the Madden 2002 cover, he threw 23 interceptions as the Vikings slumped to a 5–11 record. He also broke the record for most fumbles in a single season. While his career looked to be back on track in 2004 with a career season, he blew out both knees in 2005 and 2006 and never resembled the player he was at the start of the decade.
Marshall Faulk appeared on the 2003 cover, and his career (and the success of the St. Louis Rams) severely declined afterwards. He did not register another 1,000 yard rushing season and his yards per carry average dropped from a consistent 5.4 over the previous three years to 4.5 in 2002 and 4.0 in 2003 and 2004. He started 21 out of a possible 32 games from 2002–2003 as knee injuries got the better of him. He underwent reconstructive knee surgery in 2005 and retired that same year.
After appearing on the cover of Madden NFL 2004, Michael Vick broke his fibula in a pre-season game against the Baltimore Ravens, and the Atlanta Falcons went 5-11. Years later it was discovered that he participated in an illegal dog-fighting ring and he was sent to prison.
Ray Lewis, 2005 cover athlete and middle linebacker for the Baltimore Ravens, had his season cut short in week 15 with a wrist injury. It was also Lewis' first season without an interception.
In 2006, Philadelphia Eagles quarterback Donovan McNabb tore his ACL and meniscus in his right knee while jumping out of bounds in a game versus the Tennessee Titans, ending his season. McNabb also suffered a sports hernia in the first game of the season.
Running back Shaun Alexander, then the league's reigning Most Valuable Player, was featured on the cover of Madden NFL 2007, and sustained a foot injury that caused him to miss six starts.As a result, Alexander's rushing statistics were substantially less than those from the previous season, and he never returned to true form. Alexander himself has asked, "Do you want to be hurt and on the cover, or just hurt?"
Madden NFL 2008's cover featured Vince Young, the starting quarterback of the Tennessee Titans. Young missed one game of the 2007 season due to a minor injury, but for the rest of the season was roughly even with his 2006 performance. When Young appeared on Jimmy Kimmel Live to officially announce that he would appear on the cover of Madden NFL 2008, he derided allegations of becoming the curse's next victim, and stated, "I've done prayed about it and we're gonna go home and try to get to the playoffs and try to get to the Super Bowl. We'll see what happens."  In contrast, Young suffered longer-term injuries in 2008 (from which he successfully came back in 2009) and 2010, which, combined with a coaching dispute, led to his departure from the Titans prior to the 2011 season.
Brett Favre appeared on the cover of Madden NFL 2009 as a Green Bay Packer, under the impression that Favre was retired. However, shortly after the decision, Favre came back out of retirement and was traded to the New York Jets, where he was plagued by poor on-field performance and an off-field sexual harassment scandal.
On September 10, 2009, Larry Fitzgerald, wide receiver for the Arizona Cardinals and safety Troy Polamalu of the Pittsburgh Steelers, were both featured on the cover of Madden NFL 10. Polamalu sprained his MCL in the first half of the season opener and missed the next four games. After returning, Polamalu played in three more games before injuring his posterior cruciate ligament on November 14, 2009 against Cincinnati, and missed more games as a result. Fitzgerald, on the other hand, started all 16 regular season and both playoff games for the Cardinals. He did not attend the Pro Bowl due to a rib injury, leaving Steve Smith of the New York Giants to take his spot in the 2010 Pro Bowl. He recorded 97 receptions (third-most of his NFL career), 1,092 yards (fourth-most), and a career-high 13 touchdowns.
Madden NFL 11 cover athlete Drew Brees played in every game of the 2010 NFL Season. However, he threw twice as many interceptions (22) that season than in the Saints' 2009 Super Bowl-winning campaign (11). Also, his Quarterback Rating dropped nearly 20 points, and the Saints lost in the first round of the playoffs to the 7-9 Seattle Seahawks, the only team to date to make the playoffs with a losing record in a full-length season. He made the 2011 Pro Bowl as a reserve, after being unable to participate in the 2010 Pro Bowl, as the Saints had made the Super Bowl. Brees played 6 weeks of the NFL Season with a torn MCL, a fact that was first outed by his teammate, Heath Evans, later confirmed by another teammate, Jon Stinchcomb and, eventually, Brees himself. More recently, Brees's team, the New Orleans Saints, were implicated in a bounty scandal. The Saints lost draft picks, and head coach Sean Payton was suspended for the 2012 NFL season. Former defensive coordinator Gregg Williams has been suspended indefinitely.
Madden NFL 12 cover athlete Peyton Hillis has missed five games in the 2011 NFL season with a hamstring injury, missed one game with strep throat, and has had his numbers decline continuously after the second week. Hillis suffered a re-injury to his hamstring during practice before week nine and his outlook to finish this season with numbers equivalent to his previous year's performance is looking bleak. In addition, his divisive behavior in the Browns locker room during the season, particularly with regards to his contentious contract negotiations, has been the subject of much media attention. It has been rumored that Hillis will leave the Browns at the end of the 2011-2012 season as a free agent. When asked his significant decline, Hillis pointed towards the infamous Madden curse as a factor, saying "Things didn't work in my favor this year. There's a few things that happened this year that made me believe in curses. Ain't no doubt about it." 
EDIT: Also, as Adam Schefter points out, Peyton Hillis's curse this year may have been so strong that it affected Peyton Manning and Sean Payton.
[ Edited by 12b6demurrer on Apr 11, 2012 at 11:12 PM ]