I for one have never had any trouble with Li ion or Li polymer batteries overheating. Perhaps a qualifier to that, overheating to a dangerous level, I have found them warm / hot to the touch after a discharge recharge cycle, and I have had phones running GPX track software for extended times (screen off) get very hot in a pocket, approaching too hot to hold tightly!
Phones would be original OEM batteries, others would be a mix of OEM and third party, all behaving similarly.
I do not however doubt that a phone under a pillow could get to critical point, a good reason to charge them elsewhere than in the bedroom, and have working Smoke Alarms and an exit plan that all know and understand!
My 7yr old niece was terrified of the smoke alarm when I fitted new ones in her house, I put her in charge of testing, each time she does a pinch and a punch for the first of the month she tests the smoke detectors!
Has anyone actually had problems with a battery, Canon or generic, overheating?