Here's some new info about PS0 that I've been able to gather from around the web. I'm not entirely sure how much of this is rumor, but nothing is really confirmed until the game comes out in Japan on Christmas day.
There is online play, but it uses DS friend codes. There are no lobbies or massive multiplayer like the previous online games. You can be matched up with random players who you haven't exchanged friend codes with if you want, though. There is single player, too.
No fees for online!
The game's battle system and character creation borrow heavily from Phantasy Star Online.
Storywise, this game is not connected to any previous Phantasy Star title. It's an entirely new story (again).
Mags are back!
The character creator has 4 heads, 3 hair colors, 5 costume colors, 3 skin colors, 8 voice types, and 5 mag colors. It is not yet known if these are different for each class, but there are about 14 classes, so it seems probable.
Shields are now a weapon. You can block with them (not automatic, though) or attack with them.