I'd say that over the shorter term, Davis has a harder time than Cameron; he won't have the whole 'hug a hoodie/husky' phase that helped to decontaminate the Tory brand, so I'd imagine the Conservatives do worse in the polls up until, oh, say 2007 or so. I'd still expect Blair to stand down somewhen about the same time as OTL, and Brown might well call an early election, without the poll scare of OTL.
If he does, I'd expect a narrow Labour majority. If he doesn't, I'd say Davis does better through the second half of that Parliament than Cameron: he has a gravitas that Cameron lacks and is immune to the 'posh boy' attack that was perhaps Cameron's primary weakness. If the Parliament goes full-term, I'd expect a narrow-to-moderate Conservative majority in 2009/2010.