There are basically two stories out there.
1. Paul Krugman: "[T]here are clearly some Democratic senators who are lukewarm at best about change, out of some mix of industry influence and personal conviction." This is shared by, for example, Chris Bowers.
2. Your host at Plain Blog: "[T]he problem for marginal Democrats is that (1) they want the bill to pass, because they're better off if Obama is doing well (oh, and they may favor it on the substance, too); but, they don't want to be perceived as supporting a purely liberal bill."
First of all, I don't think we can know which of these is correct right now; as I've said, one can't get a read on true preferences while the cards are still in play. (Hey, reporters! The ones who are producing books over this -- be sure to let us know what marginal Dems really wanted).
Second, for what it's worth, I think the evidence is pretty strongly in favor of my version, which is more or less the standard political science interpretation of Member motives (that is, they care about reelection, so much so that they can be pretty paranoid about minor threats). If marginal Democrats were against health care reform and wanted to vote with Republicans, they could simply become Republicans -- outside of Lieberman, who is a very different case, virtually all the tough vote Democrats come from states that lean Republican in presidential elections, in many cases strongly enough that McCain carried them in a very good year for Democrats. If they really subscribe to Republican ideas, they'd be safer (as Richard Shelby has been, and as Ben Campbell was) just switching.
There's actually a fair amount at stake in knowing which of these is correct. If it's the Krugman version, the remedies are, first, to get rid of the bad-faith Dems because they're really no more reliable than the Republicans who would replace them; and, second, to care a lot about lobbying and campaign finance reform, because if the rules were different then perhaps "industry influence" would be removed. If what I'm saying is right, however, then what needs to be done is to make it easier for marginal Dems to support their party without risking their seats. That doesn't mean that the president and Congressional leadership should never be tough with a Ben Nelson or a Blanche Lincoln, but it does mean that it's a very tricky game to do things that would hurt them electorally, because a Republican in those seats would be (from the Democratic point of view) much, much worse.
So, yes, I think Ben Nelson does want health care reform. I just think he doesn't value it nearly as highly as he values staying in office. And he's in a funny place where having the bill pass probably helps him (because it will help Obama's popularity, even in Nebraska), but voting for that bill may well hurt him. He's not being slimy, any more than are Dems in safe seats in the House who may or many not care about the underlying substance of the bill but care a lot, for electoral purposes, of keeping liberals happy (and thus avoiding a tough primary, the only way that safe-seat Members of the House can lose). They're just doing their jobs as politicians.