100 degrees is nominal when it comes to high heat being bad for the battery. My old phone (Motorola XT720) was notorius for running hot, so I did some research about it and found that you really need to get worried when it starts to hit 50c or 120f+/-. This is when it slowly start to degrade the interior.
I got my old motorola up to 132f one time playing a wicked game of **cough**scrabble. I just backed out, closed the app down and let the phone sit for a few minutes. No harm no foul.
Just for a frame of reference, I have the MLB At-Bat app to listen to baseball games when I am at work and something about that app makes it run hot. 3rd year in a row I've had it and each year it runs hot when I am listening to a game. I have a temp widget on my home screen it and when I listen to a game it hovers around 104f. Knowing that 50c/120f is the danger zone, I don't worry about it too much.
If I can find that old thread of mine, I will post a link for you. Bunch of info in it but I can't search for it now.
"The work of men of genius, however wrongly directed, hardly ever fails to open the eyes of men to new possibilites."
- Mary Shelley