I'd like some input on this on those working with agile methodologies...

A current project is finding that development on our planned user stories is finishing some time before the end of the iteration, and that the testing effort and business acceptance is what's actually dragging us out longer towards the end. This means that the devs in question have spare time, and they're essentially going out to the iteration+1 backlog and starting work on cards there before our current iteration cards are 'done'. As iteration manager, I want to put a stop to this - I want a more team-orientated approach where the group takes ownership of getting all the cards done, as opposed to "Well, dev's done so what do I dev next?"

The problem I face is convincing the team of this. On one hand, I understand why the devs don't want to test the code they've written (there are unit tests they write of course, but the manual testing to be done could be influenced by their bias). The team sees working ahead as making our next iterations easier, because a lot of the work is done before we start. I see this as screwing with the whole system of planning/actuals - but it's difficult to convince the team as to why this matters.

What advice can you guys and girls give? How do we stop devs reaching ahead? What should they be doing instead? How much of a problem is this in the scheme of things, if things are still getting done?