If the tools you use indicates voltage when charging, please post that info along with temperature - cheers, thanks! 
Just a note: You'll see the "peak" voltage only when the phone is nearing it's capacity.... like you will likely see 4.2V around 80% of charge... if you look at the voltage at that time, it will look normal or even low.... but when the phone is near it's capacity, the Voltage should stay at 4.2 and STAY there... never exceed it. The temperatures I've noticed are 105 and 108, voltage peaking at 4385 at 97-100% charge. Currently I'm charging and the phone is at 43% and reading 95 degrees, 3.976 volts. Once it hits 4.2 volts, the temp will climb a lot and the amperage into the battery will drop a lot. If you use a car charger, they typically charge at a lower amperage (400 mah to 700mah) and that can keep the battery cooler, but most will charge at 5 Volts are higher (with the phone reducing the voltage to 4.2 if working properly)... and the extra voltage is what is frying the phone. Then, when the phone is unplugged, the battery has TOO much voltage (but the same mAh, or capacity.... roughly) so the battery drains that voltage way too fast... heating up the phone and battery. A lower mAh charge rate should help reduce the heat a bit, but add more time to the charge... but the high voltage is still what is going to do it in. These phones run great on 3.7 up to 4.2 Volts, but when past that it's just too much "PRESSURE" going through the phone, heating it up. The only thing I can think may POSSIBLY help is if you find a charger (home, car or otherwise) that ONLY puts out 4.2 volts.... then the phone has no choice but to keep it there.
I've been considering pulling the battery when it needs to be charged and putting it on my digital charger at 4.2 Volts, but then it defeats the purpose of the phone charger, or being able to keep my phone on when I charge the battery. Sprint says they will note this is happening, but I don't really know how to draw it to the attention of them and/or HTC in a timely matter... mainly meaning before everybody's 30 day phone guarantee is up.
Another option is to unplug the phone when the battery is at 4.2 volts when charging, that will stop the battery from getting to high of voltage, but then it won't have anymore mAh going into it.... meaning a battery capacity of only 70 to 80% or so of original design.
In the end, putting a slower charge rate on the phone (below the 1 Amp/1000 mAh charge rate) will definitely help with the heating issue in some ways, but once you surpass 4.2 on the voltage, it's going to heat up.
I know one thing for sure... LiIon and LiPo batteries are not to be "played" with.... when RC packs are charged beyond 4.2 volts (nearly impossible with a digital charger), they tend to get HOT, puff up, and if it continues burst and/or catch fire. RC batteries hold 8.4 Volts at 5,000 mAh so it's much more of a danger, but a fire or burst with a LiIon battery at any voltage is dangerous.
Gabe
