As pointed out by Tom Kowalski, today is the first day that teams can start releasing players under contract. For the Detroit Lions, this means that they can go ahead and part ways with linebacker Julian Peterson at any time. The Lions have already said that Peterson won't be brought back next season, so if they want, they can take care of this particular roster move today in order to just move on from Peterson.
Kowalski raised the idea that the Lions could hang on to Peterson for the time being and try to trade him. The problem with that, however, is that trades involving players can't be made until a new collective bargaining agreement is in place. There's no telling when that will happen. It's possible a new deal could be completed in the coming weeks, or a lockout may happen and force teams to be limited in their offseason moves.
The other issue with trading Peterson is the fact that he is set to make $8 million in 2011 and $8.5 million in 2012. Part of why the Lions are expected to part ways with him is because his contract is not worth paying given his play on the field, so it's doubtful another team would make a move to acquire him.