Anthony Wells has a very interesting post about the possible sources of surprises in the election campaign over at the Polling Report (a species of advanced headquarters he's set up for the campaign). Read it, it's worth while.
Broadly, his argument is that the apparent stability of the polls conceals a considerable potential for shocks, from a wide variety of sources - the end of tactical voting, a surge in the Liberal vote, a breakthrough by small parties, "events", unexpectedly successful Conservative targeting and more. I have to agree. I've said it before on this blog and I'll say it again - there is a considerable lake of freefloating, unfocused discontent out there, and the party who can channel it will win. It wouldn't take much to get hold of it - the question is what. At the moment it is bubbling around single issues - but those could always be aggregated. Note the talk of an alliance between Fathers4Justice, UKIP and the Countryside Alliance. A truly bizarre coalition, if probably entertaining. Less flamboyantly, there's the possibility of more Richard Taylor-style protest candidates from the Left (anti-war, or anti-PFI). And there's always George Galloway, who's putting his libel winnings into a campaign in Bethnal Green. Wellsy seems to think that Robert Kilroy-Silk's Veryarse..sorry...Veritas might be a serious proposition. I doubt it. (Doesn't that New York Times profile look twice as batshit crazy now..)
What it boils down to is that we're looking at a political pool of errors. "Pool of errors" is a navigational term for the situation where, not knowing where you are, you estimate the maximum distance off course in any direction you could be. This gives you the pool of errors, which you know you are in. Although there are some handrails that set the edges (there isn't going to be a Tory landslide, the marginals don't favour a Lib Dem win outright, but Blair is surely not *that* popular), the rest is uncertain.