I honestly think he has been healthy for a while. Is it just a coincident they never put Harvin on the I.R. and he suddenly was ready to start practicing right after locking down the #1 seed and getting a bye week?
I think Harvin was told to not speak (twitter, interviews, etc), not play, and heal until playoff time. The Seahawks did not need him during the regular season. Plus, if Harvin starts during the playoffs, opposing teams in the playoffs won't have much film (with him in it) to study.
Harvin was probably getting pissed off reading and hearing the media talk bad about him while he was out (especially knowing he was fine, but couldn't speak or tweet). That is why he tweeted something about "haters fueling his fire". Notice his twitter account was silent after he was "injured" during the Vikings game? He would only tweet about the holidays.
But that is just my opinion. I may be wrong.. lol.