Based on the specs, I'm guessing that the Wii (like most consoles of previous generations) simply doesn't run hot enough to have the same overheating problems as the Xbox 360 and PS3, even if you play it a lot.
That and they are using existing hardware, that's been fully tuned. Both the 360 and PS3 are using hardware, that hadn't rolled out in market at the time.
In other words, we were their testers to a certain extent. Personally, that's why I held off buying a PS3 until now (buying one in November).