Hi all!
I hope my post will help other people like myself that ware looking for information regarding this issue:
I installed TempMonitor and followed the teperateure of the device. There are 2 temperatures to measure: CPU and battery.
I will elaborate my findings:
1. CPU temp at rest without sync (push mail turned off) is 28-31C after a while (depends on the room temp).
In slight effort the CPU temp can get close to 50C but normaly is ~40-45C
It seems like the CPU temp is not that important and only when the CPU is running hot for ~10 minutes you feel the heat and it starts to get annoying. Notice that in this point a lot of battery juice is drawn.
2. Battery temp is also ~28C at rest. It gets up to 35C on ongoing effort. Since it is very large (1920mAh), you will feel the heat. I think that the battery is the main heat factor in the device and the CPU and graphic engine running in addition to the battery getting hot will cause the entire device to feel warm or even hot.
3. I noticed that at the beginning, one corner of the device (lower right) got REALLY hot and REALLY annoying at times. After 2 weeks, the heat spread became more bearable. I think it has something to do with the battery "setteling in".
4. Running apps that constantly check for updates and even push gmail, will cause your phone to run 1-2 degrees hotter and the more apps you add the hotter the phone will get at the resting state. Hence, when starting other apps it will get hotter and faster. even PowerControlPlus which is a very simple widget gets the deviec ~1 degree hotter on idle.
5. As the battery gets drained, it emits more heat. Try to charge your phones and not run battery draining apps when the battery is less than 30% or even 40% full.
I will comfort you all by telling you that I have a few friends with Galaxy S and Vibrants and their devices get hot too. A bit less on normal use like browsing but pretty much the same on 3D games and such.
I hope I helped because I was really frustrated by it for the first 2 weeks. I hope it will keep improving. I also purchased a case and hopefully it will help with the heat spreading around the phone - will update.