This happened to me last year. Came up with a high-concept idea. Started researching and outlining and just as I was ready to type "fade in:," I heard that an A-list star was in rehearsals on a movie with the exact same premise. I abandoned the project. The other one went straight-to-video, so I might be able ot revive the idea one day. In the meantime, I came up with an even more commercial idea and this incident made me write it faster before anyone else has a chance to come up with it on their own.