It was the same exact thing with Gears of War and every other game that people played a lot when it first came out, same thing with dashboard updates, just coincidence. If the thing hasn't ever been opened it's M$ problem, they extended the warranty to 3 years for the 3RLOD issues, they will repair/replace it.
With as many presses of Halo 3 there are bound to be some that are bad too, that just happens when ya deal with that many of them. Them releasing it in a crap case that scratches them all up was just sheer stupidity though and all their fault.
Murphy's Law - If something can go wrong, it will go wrong and at the worst possible time. Ya really wanted to play Halo 3, the 360 showed it's arse and 3RLOD on ya, it was just coincidence, really crappy coincidence, but still coincidence. Keep gaming on it pretty good for another couple days, it's bound to red light on ya again before long, that issue doesn't just 'fix' itself and if it's still under warranty then call up M$ for a box (coffin) for the thing and get them to straighten it out. f you've voided the warranty ten look into having it reflowed and the X-clamps replaced, plus doing some additional cooling modifications to it.