And how do you know that? You haven't done the testing required to say how many will actually fail under normal use in a well ventilated area. Let's look at things this way; both of the other next-gen consoles have failure rates below 1%. I can't say for certain with the Wii since I don't have one, but the PS3 produces a lot of heat itself. If a large portion of those 360's that are failing are due to people not letting them ventilate properly then you'd probably see about the same failure rate due to overheating for the PS3, or at least something higher than 1%. That's not the case though which implies to me that either PS3 owners are abnormally smart in comparison to 360 owners, or it dissipates the heat well and the 360 doesn't. Don't try to feed me some crap that 30% of 360 owners are complete morons when failure rates for other consoles are so low in comparison. The console wasn't built well. Obviously it's failing to dissipate heat well under what could be considered typical use by your average consumer, and this is leading to a lot of hardware failures.
I'm not saying this because I have a problem with the 360 as a gaming platform. I'm saying this because it's poorly built. The proof is in the numbers whether you want to admit it or not.