I called them up and found out some things:

-Too much international traffic may cause your account to be banned.

-There is nothing in between, ie they won't send you a warning message stating that you are getting too much international traffic. Once it hits a certain point, your acount is banned.

-It can take up to 45 days before this determination is made.

I have a site where people from everywhere come to visit so now I'm rethinking putting up so many YPN ads as there is no way for me to know if I am sending too much international traffic.

In response to this U.S.-only requirement, I have restricted YPN ads on one site to just a small handful of pages with content of little or no interest to foreign visitors. (On two other sites, all pages are potentially of interest to a world-wide audience, so no YPN ads at all on those sites.)

Moreover, on those few pages, I have chosen an ad category (telecommunications providers) that should be geo-targeted to U.S. audiences. (Unfortunately, general interest ads with potentially world-wide appeal still show occasionally.)

So, I am displaying single-category, off-topic YPN ads on a small collection of low-traffic pages on just one of my sites. This effectively cripples my participation in the beta program. I might avoid the non-U.S. visitor ban, on the other hand, Yahoo might disinvite me for reasons of low traffic.

Sigh. I can't wait for YPN to go fully international.