Today he declared that he'd fulfilled that pledge. By not leaving the EPP. Until 2009.
In itself, this is pretty much irrelevant - the Tories are simply looking to create a new, less-federalist group and need the support of another handful of countries to do it, or they face sitting on the sidelines as outsiders. Cameron made the deal, not realising that the initial plan would require the Tories to ally with people like the Polish Law and Justice Party, which has rather old-fashioned views on equalities and homosexuality - entirely out of kilter with the New Tory ideology. He thought the Czechs would play ball, but the Civic Democrats are suddenly finding that they need their European friends, so aren't quite so anti-Brussels as they were. Cameron knows he's in a bind and he hopes that this will delay action for three years, by which time an election campaign will have either been won or lost. If it is won, then Cameron will be PM and can say that times have changed since the original declaration and the time isn't right to leave the EPP. If it is lost, then the new leader of the party will have to deal with it.
But the truth is that despite whatever the loyalists say over on Iain Dale's site, Cameron has broken his promise. In any case, that promise would have itself broken a European election manifesto pledge to stay with the EPP for the 2005-09 term.
Indeed, back in January, any MEP who failed to leave the group was threatened with deselection. Now, any MEP who LEAVES the group will be deselected. As he put it on the World at One on the 16th January:
Let's face it. If you voted for Cameron as leader thinking he'd keep his word, you've been had.
'I've said that I think the Conservative Party ought to leave the European People's Party because I believe in consistent politics... and you'll have to wait and see for the steps... we're going to take together to deliver this important promise, which is about being consistent.'