From YPN invite application: Participants of the program must have a valid U.S. Social Security or Tax ID number, and web site content that is predominantly in English and targeted at a U.S. user base.
Translation: everyone with an English language website targeting the U.S. can apply - just set up US company first if you don't have a Social Security Number of Tax ID yourself. An LLC will do just fine, 100$ to set up. A little more if you need a trustee to fill in the form to get the Federal Tax ID.
Anyway, taking into account all these hotheads who might suddenly sue you for no apparent reason, I would be very hesitant to do business in my own name these days. I have a U.S. taxpayer ID (as a person), but I still prefer to use a company for YPN - or any other business.
Since I'm at it - for all those who scream about TOS when someone is "selling a YPN account": yes, you're not allowed to sell a YPN account. But Yahoo will not prohibit you from selling your company which owns a YPN account. The YPN account is never sold, but the beneficial owner changes - in full compliance with Yahoo TOS.