I doubt MS would do anything less than a TV announcement. Most people don't even know about GDC.

Microsoft is not ready yet, simple as that.
Software and hardware both are incomplete.

GDC is too small for a console announcement. I expect it to be a Sony-esque event.

GDC is not about consoles anymore. This is 2013 guys, it is all about Tablets / Smartphones / Browsergames. You guys are overestimating GDC.

Check this article:

http://www.businessinsider.com/why-google-should-be-afraid-of-samsung-2013-33

Most interesting part:

it's pretty obivious that game developers are under NDA contracts.. are people really worried that the next xbox won't have game support? the high end 3rd party market isn't sustainable if they would only release ps4 games in the future...

they obviously won't talk about it because they haven't announced it yet. i don't think Ms can be arrogant they haven't had nearly enough success in 2 gens compared to Nintendo and especially Sony in there first 2 gens.

MS no doubt keeps in touch with devs. They just dont want us to find out yet. No biggie.

Wow, Nextbox (slightly) above PS4 although not even shown? :) good 3rd-Party-Support  for NextBox confirmed ;)

Thanks for giving us a spot to your proficient knowledge and wisdom, Sir. Next time we can ignore rumors and listen to your wise words. 

I don't think there's anything strange about the 720 not being there. I do think Microsoft not being there at all is a bit strange though, I thought they may have wanted to push Windows 8 development as hard as possible, and development for their phones and tablets.