Hate to say it, but I think we're remarkably close to getting screwed by Bush and Holy Joe, and we're not even thinking about why. Lieberman found himself ignominiously rejected during the 2004 primaries, basically ignored during the election, branded a traitor during the Gonzales vote, and then viewed as an enemy on Social Security. The sum total of all that has been a marked uptick of interest among Democrats in finding and funding a primary challenge against him. Worse, Joe's got nowhere left to go, it's unlikely that Democrats are going to retake the Senate anytime in the near future (which would give him a committee chairmanship) and it's damn near impossible that he'll be on another presidential ticket or in a hypothetical Democratic cabinet.
With all that in mind, I see no real reason he'd want to languish in the Senate, condemned to a future of intraparty battles and partisan marginalization. Cutting a deal on Social Security might be his way out, because it might bring with it a new position for Joe: Secretary of Defense in the Bush administration. There's little doubt that he's more interested in war and peace than Medicare reform, and by this point his only chance at respect is being the very embodiment of bipartisanship. Taking credit for shattering the partisan impasse blocking Social Security reform and then ascending to a top cabinet position seems like a pretty attractive path for the otherwise marginal fence-straddler...
Update: I should clarify that I find it entirely possible, given Bush's history, that the deal will be stuck, Joe will will vote against SS, and then he'll be left twisting in the wind. My point in the post is not what Bush will or won't do, but what Lieberman thinks he will or won't do.
(If there's one thing we know about comment trolls, it's that they're lazy)