I don’t think nominal control of the Senate is all that important (which is why my concerns about the O’Donnell nomination have not included that it endangers that “control”). In the House, 218 votes really is a magic number. Having a tiny majority brings headaches but going from 217 to 218 is a much bigger increment of power than going from 216 to 217. I don’t think that going from 50 to 51 in the Senate is quite as crucial. The key numbers in the Senate are 40 and 60–and even that gets fuzzy if party discipline does.
I’d fiddle with FirstRead’s rankings on the likelihood of Republican pickups. I’d say Colorado is more likely than Illinois, and Wisconsin more likely than Nevada.