I have been following the spread betting scene for some time now. Head over to Cantor Spreadfair or IG Index to stake on the election. The interesting thing as pointed out by Chicken Yoghurt is the punters seems to be believing a Lib-Dem outcome of 65-ish rather than BBC's exit poll of around 50-ish. And looks true from most of the spreads I have seen.
If you have a PDA with constant connection, you can even use IG's Mobile Dealing service to play.
I am not really a betting person but I did stake a couple, not a spread bet though.
Update (1am): Hmm...the spread betters are now buying 45-50-ish Labour majority.