Look like, Yahoo will never out of beta. Yahoo will dead in beta version. So, waiting for yahoo publisher is useless
The probability that MSN's AdCenter Publisher is more likely now to come out of beta than Yahoo, even though AdCenter is a much newer platform. I hope MSN AdCenter does go international and the same with Yahoo, would add a lot of awesome potential to online publishing for webmasters.
I don't think YPN will go international ever. Yahoo targets the US from day one and even after google invaded the zeros and ones world, many Americans prefer to use Yahoo (not only for the search). Thus, YPN is much valuable within the US. Also, cheating control is much easier this way.