I believe it's Ad-Hoc only.
However, if you have a PS3 and a Japanese PSN account, you can download a special add-on for the PS3 (JP Store only for now, I guess?) that works similar to X-Link Kai, a tunneling program that "tricks" your PSP into thinking it's connecting with someone via Ad-Hoc when it's really not. A pseudo-infrastructure I guess.
I think it works like this:
PSP Wi-Fi connects with PS3 (which is hooked up directly to the internet via ethernet cord) and allow you to connect to other PSN members who have the game (and the Ad-Hoc add on) I think..that's it? Not too sure.
Anyway..back to the game..I haven't played much but so far, it's really interesting. I'm a bit sad I won't understand what the character's are saying but the battle system is pretty fun. I'm a fan of Onion Knight..that little bugger is really fun to play with.