Another issue with Peterson is that his yards per attempt have steadily declined:
2007 - 5.6 ypa (2nd among backs)
2008 - 4.8 ypa (5th among backs)
2009 - 4.4 ypa (24th among backs)
In the playoffs this year he really declined (3.6 per attempt)
Source: ESPN ( To qualify, a player must have at least 6.25 attempts per team's games played)
The drop-off from year 1 to year 2 can largely be attributed to defenses having had a year to figure out how to stop him. His first year he had some huge games simply because defenses weren't prepared for a rookie to come in and dominate like he did.
Year 2 to year 3 is a little misleading because he got a new quarterback and a new offense, resulting in his rushing numbers being diluted a bit. Note that if you add in his 10.1 yard receiving average, his yards per touch are actually up from last year.
I don't think this is necessarily something to put on Peterson. Over the years, the Vikings have added more offensive weapons, notably a better receiving core and aquarterback. With Brett Favre there this year, the Vikings had a passing threat that was lacking with Tavaris Jackson, so Peterson wasn't relied on as much to run the offense (This is similar in San Diego with LT - his role is reduced now that Philip Rivers has become a better qb). I also think that defenses are more prepared for him than in previous years, when he was first breaking out on the scene.
In the playoffs this year he really declined (3.6 per attempt)
Source: ESPN ( To qualify, a player must have at least 6.25 attempts per team's games played)