Also opened it up as a Quora thread if you want to answer anonymously - http://www.quora.com/Y-Combinator/Foreign-YC-Founders-where-did-you-live-during-the-program
Strictly speaking, B1/B2 doesn't allow you to work. I have a visa called B1 in lieu of H which allows you to do H work without taking a salary, of course. It's also not tied to a particular company but to an individual. I have no idea how I got this visa because I applied for a regular B1 and the annotation was added to the visa – it also isn't a widely recognized visa (embassy issues it) and I would suggest against applying for it. I guess they gave it considering my history of startups.
EDIT : ok found it, it's right in the FAQ, must have been reading it again a few days ago ^^ http://ycombinator.com/faq.html under "Can we do it without moving to where you are?"
If you have an existing company that's been around for a bit you may want to look into L1 visas.
Plan on moving here for a while as a founder.
Yes there is the Visa Waiver Program for people who can so, but assuming you are working on your company, I doubt that technically legal.
From my understanding of the law, you cannot enter the US in YC unless you don't actually work for your company. Then again, in reality you can probably get away with just staying wishy-washy (for which I would be still curious what people have said: Im going to learn at a program? Going to visit some people?)
You do have to be in California for most of your time, just because you wouldn't get much of the value otherwise.
Thanks for the replies everyone