I'm not trying to get involved with defining what a hackathon is (or isn't), but I do know that Startup Weekend (www.startupweekend.org) has been hosting events that have prizes and winners for quite some time now.
The Startup Weekend I attended in January had winners but it was never the emphasis, and the prizes weren't anything special anyway. I liked the presentation/judging process was nice because it gave you an end goal for the weekend, and it was interesting to see what the judges said about your project.
