Thanks for all of the great comments - I'm still going through and replying to them.

Another thing to keep in mind:

I've spoken vaguely about the site, and how it works, but nothing specific. They'd like me to come down, and discuss what I'd be doing, and go through some of the code behind the site. Is there any way to protect myself from them just copying my idea, and stopping talking to me? I signed an NDA, but that just protects each of us from discussing how the other's business works with people outside their business. Is there any way to protect myself from this?

I pretty much asked them this exact question, and they basically said 'they talk with a lot of companies, and if they did so, word would get out, and no-one would talk to them', which is true.