Michael Shear has a good rundown of conservative unhappiness with the tax deal, none of which is apparently going to affect the Senate vote (as I write, the bill is sailing through a preliminary procedural vote).
The model for danger ahead, however, is with the House Republican conference. I can't help but think of two other times when a consensus deal came undone when House Republicans deserted at the very last moment: the George HW Bush/Jim Wright deficit cutting deal, and the first TARP vote. In both cases, when push came to shove, Republican Members of the House revealed that the one risk they weren't willing to take was breaking with movement conservatives.
I have no prediction. It's certainly possible that conservatives are backing away from the deal in part as a bargaining step -- with House Democrats asking for a better deal, it makes sense for conservatives to emphasize that they're not desperate for any deal that will get them what they want on high-end taxes and (high-end) estate taxes. I'm just saying that I'm going to be watching the House vote very closely. I don't think there are a lot of examples of House Republicans defying Rush Limbaugh on a high-profile vote over the last twenty years, and if Rush is joined by enough others (yes, it's worth watching Sarah Palin), it's going to be a hard vote for House Republicans. My guess is that the one thing we won't see in the House is the vote just missing...if it can't get a majority, both Democrats and Republicans will switch away from the deal, and it'll lose by 100 votes or more.