Obviously enough, the YPN http agent is called .. Overture. Mind you, not the usual lengthy hhtp agent like "FAST-WebCrawler/3.8 (crawler at trd dot overture dot com; ".
Just a plain simple "Overture"
Also, by mere trial I found out that the "publisher.yahoo.com" page (where one signs in to be informed about the program) also appears at the address ypn.yahoo.com
Seems like this forum will have to be renamed soon :-)
