I guess it depends on what counts as a surprise. The signs are that the election is close enough for almost any result to happen; a few tiny fluctuations here and there and we could see even a landslide victory for one of the main parties, based on a set of narrow wins. More likely for a Labour landslide than a Tory one, I'd say, because of the situation in Scotland leading to a 30-odd seat swing in their favour if the SNP fall short (at this point fairly unlikely).
Two things I do not expect, and which could therefore happen, would be the Lib Dems gaining seats and -- separately -- UKIP establishing themselves as the bona fide third party ahead of the SNP and Lib Dems. I don't realistically see UKIP overturning the majorities they have to deal with in more than a handful of seats (by-elections are just a chance to stick two fingers up to the incumbents, so those don't really count), so I'd be very surprised by that and it would be unexpected to me. Equally, the Lib Dems seem to be unable to defend their record successfully so I doubt they'll be doing well either.
With just about anything possible, though, there's little that could happen that has been totally unforeseen. Roll on the election proper.