The one thing that struck me the most when visiting Lean UX was how many people still have trouble understanding how (software development) teams in the agile era should work. The most common questions: what will my testers do in these short iterations when developers are still working on features from the backlog. And what do developers do when they are done and hand it over to a tester?
C'mon, you're a team! Are you in the same room? Are you talking to each other? Did you create the definition of done as a team (or did someone do it for you?).
Forget about roles, forget about titles.
Together, as a team, you are responsible for the result of the iteration / sprint / whatever. If your main focus is testing, prepare the tests while the feature is still in development. Run these test before, while in and after development. It's the shortest feedback cycle possible! And developers, what is "done"? You are not done before all tests pass. It's not a sin to do some testing work yourself, beyond the usual unit testing. Maybe you can write tools that support testing. It's fun.
You're a team. Do stuff together. Take responsibility. Together.