I have done some research in this area, and this is how far I have come:
First of all, my phone also got really hot in the beginning (it is now 1 week old) and it really didn't matter what I did (explorer, sms, phone call) it got really hot in the bottom area of the phone.
1. I deleted 3 third party apps that made the phone really how when I used them. The phone got less hot, but still a bit to hot imo.
2. Reset to factory setting and didn't update my apps. The problem was pretty much gone and the phone was just a bit warm when doing some pretty hard work (and that's totally OK).
What I will do next it to manually update one and one app until I find that one ******* application that makes the phone hot, but the reset of the phone itself might have done the trick.
Another thing is that the phone itself have been less hot every day since I got it. Now it only gets a little warm, and it took a week of hard use. The battery is also getting better every day.
I do believe that that some apps are responsible to the overheating stuff, and if you use a third party app that makes the phone hot, try to delete it and use the phone further on without it to see if that helps. If your on your first days of using the phone, give it some more days. There is a lot going on the first days of use, and that might be a factor that many don't consider.
Just my two cents, might be wrong but I am certainly seeing improvements every day. Really happy with the phone, great design and so nice to use.