I talked to the YPN folks when I was at PubCon because I'm a Canadian and I'm not eligble. I guess if I had a US tax ID or something they could help me, but I don't. About 85% of my website visitors are from the US, but that doesn't matter.
So I'm still on the AdSense bandwagon. It's Yahoo's loss IMHO. I set a record with AdSense the last two months in a row. If YPN wants to miss out on my traffic, it's their loss. Yay AdSense! If YPN opens their program up to Canada, I'll try them out, but until then I've decided to not worry about them.