Grist has been full of pro and con debates from enviros about the Waxman-Markey (one of the later salvos here). Just thought I'd add that it's pretty obviously the only game in town, and if it fails, it's unrealistic to expect anything else this year. Doing something is a whole lot better than doing nothing.
The only reason I can think of for holding off is the hope that next year, EPA will have come up with CO2 regulations that scare the bad guys enough to cut a deal in return for legislation that makes EPA back off. Too many problems with this, though, mostly problems of delay - EPA may not finish new regulations for over a year, and then the litigation will start and delay things even longer. Only after they've lost all the battles with EPA will the bad guys cut a deal.
A better approach is to get the best legislation that we can pass by 60 votes, now. When EPA issues draft regulations in a year or two, the polluters can try and strike a deal, but the baseline that they will have to improve from is the halfway-okay Waxman-Markey, as opposed to the present situation of nothing.
I don't think the relatively-smaller enviro groups opposing W-M have much of a leg to stand on.
UPDATE: a recent Nature Podcast commentary (not sure if it was June 4 or June 11) had another good point - if the US walks into the Copenhagen conference after the end of the year with no climate legislation in place, we will have no response to India and China saying they shouldn't put limit on their emissions either. If I were conspiratorial I'd think the polluters were cooperating on this tactic.
UPDATE 2: According to Mother Jones, the current version of W-M strips EPA of authority to regulate coal emissions for climate reasons (apparently not other aspects of climate change though). Consider part of my argument shot down, then.