The team is in an interesting situation now. The trade deadline has already past, so the Dolphins must either keep him on the roster or release him. If they keep him on the roster, they can try and work out a deal with another team in the offseason.
I am all for stashing him on the roster until the season is over. I'm sure someone would be willing to part with a seventh round pick for Ricky, and the Dolphins may be lucky enough to snag something as high as a fifth-round pick.
Also, remember that Ronnie Brown must make his way back from an ACL tear. He may not be able to go at the beginning of the season. Should Cam Cameron and Randy Mueller welcome Ricky back, they could put Ronnie on the PUP list for the first six weeks and let Jesse Chatman and Ricky form a one-two combination.
[Update]: The NFL released this statement today regarding Williams:
“Ricky Williams was notified this morning by Commissioner Goodell that he has been reinstated as an NFL player. Williams can begin attending team meetings and practicing immediately, but will not be eligible to play in games until Week 12 when the Dolphins play at Pittsburgh (Monday night, November 26). The Dolphins will receive a roster exemption for Williams for up to two weeks when he begins practicing, starting no later than Monday.”This means that Miami does not have to make a decision immediately regarding Ricky. In my opinion, even if they plan to release him (which I am not in favor of), they should still hold onto him for the entire two week roster exemption. You never know what could happen in that time span. Another one of the team's running backs could get injured, and then room on the roster would open for Ricky. It would also prevent any other team (New England, I'm looking at you) from picking him up early.
In any case, I think the best move would be to keep Ricky for the rest of the season, and let him get back into game shape. Then showcase him during the last month of the season to drum up interest from potential trading partners during the offseason.