Or not! Sega has announced a Japanese PSU port for the PSP, but its multiplayer features are only ad hoc -- meaning it works over a local wi-fi connection between PSPs, not over the internet. So they've taken PSU, stripped out the "Massively" and "Online" parts of "MMO," and ported it to the PSP.
Bad form, Sega -- bad form. Looks like we'll be waiting a bit longer for an MMO-on-the-go that isn't a cell phone game meant for 'tweens. Maybe NCsoft is PSP MMO fans' only hope.