pffft.. screw startups, go work with the best people in the business on one of the best projects out there, Bioshock 2. 2K Marin are looking for level designers. http://www.2kmarin.com/ tell em Jay from 2K Aus sent ya.
As many before me have said, it's better to start a start up when the opportunity and market timing are right, not when your timing is right. In the mean time, working with a bunch of performers with a proven track record isn't a bad alternative.