Last two polls with a unified "coalition" ticket:
YouGov: Coalition: 352, Lab: 275, Other: 22
ComRes: Lab: 321, Coalition: 304, Other: 24
That clearly seems to be the way to go for Cameron and especially Clegg.
Conservatives wouldn't actually win that much from a Coalition ticket IIRC, while the Lib Dems obviously would be able to hold on to quite a few seats (not in the least those seats where the conservatives currently are their main challenger). On the other hand a coalition ticket would probably have every single Left-leaning Liberal backbencher calling for Clegg's head. So that means Cameron hasn't got that big of an incentive to go for it, and Clegg's hands are tied by his own party on the issue, making a coalition ticket not very likely IMHO. Maybe one of our British posters can elaborate?