I have just gotten off the phone with my friend, who works with the Sonic team. This person, who shall remain annoymous (at his request of course) has informed me that the Sonic Team is going to develop Phantasy Star Online for Gamecube! This confirms that the Sonic Team IS developing for other consoles, and the Gamecube will have it's own online network!
My friend has also informed me that Sonic Adventure has not been pushed back to the fall, but rather has been pushed ahead to January 30th, 2001! Expect an article about this on IGN DC when it opens back up after the new year.