Had the exact same thing happen to me. Two days after getting MW2 screen goes green/blue and switches off after a couple minutes. This is on a launch 360 that has been sent in for repair once. Said stuff it, bought a Jasper Elite, love how quiet it is. Plus I now have HDMI for added neatness...
Wow, I haven't had a single problem with mine since launch day, I keep it well ventilated and stands vertical or horizontal depending on my needs. Never hiccuped ever.
Good luck with your return though.
Just lucky I guess.