I'm betting that in the age of Apple, tech makers see the value of big announcements in a spotlight of their own. Sony had a very successful conference and generated enough buzz to dominate the games media this week and weeks to come.
Microsoft will reveal the 720 well before E3, and away from GDC and Pax East if only to curtail the momentum Sony has built. E3 will be used to focus on software and new IPs.
This holiday season will be a console war like consumers have never seen. 2 next gen consoles on the market fighting for your wallet. This will be interesting.