You can bet Cameron is on the phone to Prague trying to see what they can do to not ratify the treaty in order to save him from possible extreme embarrassment.

If the Czechs pass the Lisbon Treaty, then Cameron is left with egg on his face, as he cannot pull the UK out of just the treaty after it is ratified. The referendum would have to be a referendum on the entire future of Britain in Europe, which is a debate that the Tories really don't want to get into, as it's an area in which they are weak politically. If the main issue of the election is the economy, the Tories will win handily. If it moves to Europe, the debate could suddenly turn against them (either completely or at least to the point of a hung parliament and a Lib Dem-Labour alliance), lest they be forced into adopting the arguments of UKIP or the BNP, the extremism of which (and much moreso the latter) would probably drive people back to voting for Labour or the Lib Dems.

OTOH, Cameron has promised the people and his party a referendum on the matter. If he turns around at conference this weekend and says "No referendum," (and from the sound of it he won't, instead trying to stall the issue until the Czechs and Poles decide what they want to do) the party will be extremely annoyed, although they probably won't try to replace him this close to the election, and there is no viable alternate candidate, with George Osborne still tarred from the expenses scandal. And of course this gives his opponents ammunition for saying that he is not keeping his promises.

Whichever way this goes, Cameron could be in for some hot water in the coming weeks.