My box just got the RROD 2 days ago. I swear they have a button at microsoft that they press to trigger this on random machines across the world so they can shell in money for repairs.
From what I've read it occurs when the motherboard overheats, then bends and snaps a connection somewhere. You'd think they'd use wires that could take the bending.
If you do send it in, make sure you only send in the xbox, not the hard drive or any other gadgets, because I've heard of tonnes of stories where people send it in with their HD or whatever and get it back without, Microsoft just tells them it was lost in shipping on the way to Microsoft or something like that.
I've also read that once you get the RROD repaired, its destined to break again after a few weeks.
Final piece of info; theres an urban legend that you can temporarily fix the RROD for around 4 days by wrapping your xbox in a bunch of towels, turning it on causing it to heat up a lot, leaving it to "bake" for 20 minutes, unwrapping it, turning it off, waiting a few seconds, then turning it back on, and it will work. I haven't tried it yet, I really doubt it'll work, but then again there are videos on youtube showing it working, but of course that could just be some smooth camera tricks.