http://news.ycombinator.com/item?id=1778985: > It's not a deal-breaker unless it's botched. The right way to do it would be to use a convertible note written so that YC funding wouldn't trigger conversion. If you have a lawyer who can arrange that. But you'd have to incorporate, and you (and we) would then be stuck with however you did, so we'd hope you did it right.
According to those and other previous comments from him, they prefer it if you leave the incorporation process up to YC, since they specialize in setting up startups, but if you've already incorporated (or even taken funding), it's not a deal-breaker -- unless it's been done in a way that's incompatible with YC investment.
In the event that you do get into YC, or raise 5-6 figures of seed money, you can always go to a lawyer and reissue stock certs, articles of incorporation, etc.
just my 2 cents.