PS2 and PC owners will be able to play Phantasy Star Universe online on the same servers. Today SEGA announced that X360 version of PSU will not be able to do that. This means that X360 players will have to settle with playing with each other online. The likely reason for this is the closed architecture of the Xbox Live servers.
X360 version of Phantasy Star Universe will feature a huge single player RPG adventure divided into multiple planets and a squad based Xbox Live multiplayer with hundreds of players online.
PSU is scheduled to be released this autumn for PS2, PC and X360.