If the Lisbon Treaty is still being ratified by other countries (Poland and the Czech republic) at around the time of the May election, then the Conservatives will hold a referendum for the country. Fine, but that situation will not happen.
If Poland and the Czech republic vote for the Lisbon Treaty and the deal is sealed, the Conservatives say they will have to look at this again, although in principle, they would like a referendum.
Now, what would happen if Dave Cameron said:
If the Lisbon Treaty has been ratified, then there is no legal way or us to undo it. We're stuck with it thanks to Gordon "Courage" Brown. All we can do is try and bargain some sovereignty issues similar to what the Germans have done but that's about it. If we tried to undo the Lisbon Treaty it would mean our expulsion from the EU in its entirety i.e. we would not be able to trade with one of our biggest partners, forcing us turn to the USA for help.