The battery in your phone will get pretty warm. Mine regularly gets to about 112F. I have a Trident Kraken case on my phone and before I got it would only get to about 109F. Under high demand situations the battery will get pretty warm, I am using the stock battery BTW.
I have read that the cut-off for high temps is 140F (I think), but the phone will stop charging the battery if it gets over 120F (someone correct me if I'm wrong). But I have only read 1 or 2 extreme cases where the phone got that hot. I'm talking people using the phone for GPS navigating in a car, on a hot summer day, in full sun, on the dashboard, while charging. It takes some outside force to get the phone that hot.
What I'm getting at is, your phone will get hot. This is normal. It has temperature sensors in it and if it gets too hot it will warn you it is getting too hot. If you don't do anyting to cool it down, it will shut itself off. Don't worry about a hot phone.
But do some searching on this forum. This has been discussed a lot.
