An interesting discussion over at the OnStartups stack-exchange:
I was interviewed by a lady from Zynga and she told me that Zynga doesn't allow developers to have side projects. Is this true for companies in general, or just a minority that Zynga is a part of? Is this sort of restriction legally enforceable? I'm in California, for what it's worth.
The HN discussion is interesting, particularly this brief comment thread by startup lawyer George Grellas. If you are working for one company while starting up on the side, this is critical information.