My T40 is never off - it is either on or sleeping (standby). (It will go to hibernation if sleeping with no AC after 6 hours, but that rarely happens.) I carry it in my backpack with no extra padding and all ventilation blocked for 10-12 hours a day, and I've never seen it so much as get hot to touch when sleeping. When on and in the backpack, it may get hot.
Concordantly, standby doesn't consume much power at all - I once let it sleep for 2 days on battery and it dropped by less than 50%. Also, waking up from standby is an order of magnitude faster than waking up from hibernation - 1~3 seconds versus close to 30.
Interestingly, I once let it overheat and shut down while on, because it was in a car under direct sunlight. Apparently the CPU temperature sensor was tripped. No lasting damage occurred. This is what will happen if overheating does occur; this was in the middle of a desert at noon in summer in a car with direct sunlight, A/C off and fans directed away from the laptop. I doubt any backpack will provide this kind of environment.