100 degrees Farenheit is nothing to worry about. My phone heats up to that while charging.
Scanning one of the threads someone linked, max temp on the droid 1 was something like 105*C, which is damn high.
100*F = 38*C
100*C = 212*F
Processors are hot little things, and the inside of your phone regularly being 30-40 degrees hotter than ambient temperature isn't too far of a stretch right? So, unless your backplate is literally too hot to handle I wouldn't worry.
edit: mine is sitting unplugged at 89*F and I haven't touched it since last night.