My understanding is this - if you don't report by the 8th of August, you lose a year of NFL service - which affects restricted free agency, pensions, minimum salaries, and a few other things. However, you still can run off a year of your contract up until week 10. They seem contradictory in principle, but they govern different things.
Since there's a possibility of an uncapped year in 2010, players would need six years of service to be UFAs instead of RFAs, rather than the usual four. Peters, though, is under contract, one way or another, until 2010, so he's not at risk.