There are a few stories that anecdote such occurrences, take Zuckerberg vs. uConnect for example.
We thought about breaking up the development functions into separate chunks and spreading them out between different developers to minimize the chance of this happening, but it seems this would be an arduous task, and in the end, all the functions would not be integrated well, and the developers will still have a pretty good idea of what's being developed.
For anyone who has resorted to outsourcing their software development, how did you ensure the developers don't run off with the code and launch a copy of your site after getting paid?
If it's not, then stealing the code won't give your competitor any particular advantage.