Look at the possible options for Mankins:
1) The Pats release Mankins from his tender and he becomes a free agent
Ain't gonna happen.
2) The Pats trade Mankins to a team that will give him a long-term deal
Also ain't gonna happen. That time has come and gone. The Pats could never get equal value for Mankins now.
3) Mankins sits out the whole (or significant part of) the season
Unlikely to happen. Once Mankins doesn't report by August 10th (unlike past years, reporting after week 10 doesn't make a difference), he loses 2010 as an accrued season. Not a big deal since the next CBA is unlikely to keep UFA at 6 years.
This scenario isn't likely to benefit Mankins at all. In addition to losing his 2010 salary, he is unlikely to get the salary he is looking for by missing a significant number of games. While he is still a top OG, he hasn't shown elite skills since his choke job in SB 42. He needs to put a big year on his resume, not a bunch of DNP's.
4) Mankins signs a long-term agreement with the Pats
This isn't happening until the next CBA becomes a little clearer. Since that is unlikely to happen any time soon, that really doesn't affect Mankins short-term decision making.
5) Mankins reports by Week 1, possibly with some cash and non-franchise promise
IMO, easily the most likely scenario. Mankins got screwed by the last CBA. He can get angry and pitch a fit about it, but that won't change anything. He just needs to get what he can, have an All-Pro season and cash in big time with the new CBA.
He knows that now but is holding onto the chance that the Pats interior line will be decimated by injuries, boosting his leverage. If that doesn't happen, his leverage disappears and his focus turns to setting himself up for 2011.