My personal experience tells me that product development is the last thing that a start-up should outsource. You don't outsource your core competency, and the core competency of a start-up is product development.
In fact, I've seen some start-ups fail quite badly after outsourcing their development (and, in my opinion, that was because they did so). By contrast, every start-up that succeeded spectacularly seems to have had some serious engineering talent involved right from the start.
However, to keep an open mind, I'd be interested to know whether anyone knows of successful start-ups (i.e.: made a good profit, and are reasonably well-known in their niche) which did successfully outsource their product development. It'd be particularly interested if there are any such start-ups which actually outsourced their development to a low-cost location like China or India, and still succeeded.
Thanks for any information on that matter!