After a week’s delay, the adhocParty beta is now ready for use. PSP owners in Japan (not sure if it can be used outside the country, will try later) can go to the Japanese PSN on their PS3 and download the adhocParty application to start fighting monsters with their friends in Monster Hunter Portable 2nd G, the first game that will support the feature (Phantasy Star Portable is next).
Update: Service works outside Japan, and it’s easy to join/set-up a game too. Played a match of MHP 2ndG with a HR9 hunter from Japan, fun!
Update 2: I made a little guide here
Source: Press release