The Lisbon Treaty

There's something nagging me about Cameron's "well, we can't really undo international treaties" line that he's been selling. Does this mean that a future Labour government could just sign a treaty with various Labour friendly governments like Cuba or Venezuela to send them a couple of billion per annum to each, and the Conservatives would just accept it and not be able to undo it?

Then again, Cameron and most of the Conservatives refused to wait and vote for Bill Cash's sovereignty amendment, so we know how much they really care about this matter.