SEGA's announced Phantasy Star Online 2 to PC in 2011.
It was announced via livestream.
In the video posted below, you will be shown a Phantasy Star timeline featuring all the titled released for the game.
At the end, it announces Phantasy Star Online 2 for PC in 2011. It doesn't mention PS3 or Xbox 360, but it doesn't say it's a PC exclusive either.
We haven't received a press release on this yet, but we'll mail SEGA and see if we can find out a bit more on this for you.