GDC is all about game developers talking to other game developers. It's rare to announce new video game consoles at GDC. As far as I am aware I don't think Sony or Nintendo are pushing their next gen systems hard at the even because it's not about that.
MS liked to announce their hardware in big overproduced events. The Xbox 360 was first shown on MTV for Pete's sake. I expect MS to very soon start leaking real details and have a mid April television debut on MTV or Spike TV. If not TV than I think they will have some kind of big party and stream the event on XBL. MS won't talk much about tech or show a lot of tech demos, but they will show teasers and trailers for games and list off all the crazy features they know will sell to consumers.
MS knows they aren't sell the Xbox 360 to tech geeks who want to know every detail about the hardware, they are selling it to people who want the best games while they sit on their couch. Those people will watch a TV special or a brief XBL stream, but they won't sit through a two hour boring shareholder sales pitch.
When E3 comes around that's when MS will talk directly to shareholders and retail because that's what E3 is about. GDC is about devs talking to other devs. PAX is about gamers and that's why MS has Halo there in force.
