There isn't a one-size-fits-all solution for this. Once you are accepted, it helps to reach out to the mailing list(or other sources) to figure this out as soon as possible.
I was in the summer 09 batch, thing may have changed since then.
I guess many foreign founders will be from countries that are part of the visa waiver program, but my understanding is that it doesn't strictly allow working (as opposed to business meetings) within the US, which would seem to rule out being actively involved in a YC startup.
Of the successful founders I know personally, each and every one is incredibly risk averse in anything ancillary to the startup itself.
Under that assumption, trying to think of answers isn't any better than a random guess. And I was asking the question for that specific situation, even if it's unlikely, just to know how things could potential be. Furthermore, in the case the government doesn't know about your staying illegally, you would be the same as any random non-citizen trying to immigrate to the US, and it's fairly safe to say that the task is non-trivial (paperwork-wise). In that case, it would still be helpful to know if a founder-but-can't-tell-government would have any advantage at all.
Not every question can be answered by just thinking hard enough, especially those related to laws - that's why there is a whole profession dedicated to it. And it will serve you well to be more careful before going passive-aggressive.