I tried to find the origin of the problem, here's what I came up with:
When putting the Phone on Speaker Mode during the call, the problem didn't seem to occur, I managed to hold a call for about 19 minutes without a reboot. While doing this, I keept the screen from going dark by touching it regurlaly.
However (!!), the moment I put my finger over the 3 sensors on the top left on the Phone (those being responsible for locking the touchscreen while having the phone at your ear) the phone instantly froze and restarted.
So using the speakers on calls might be a temporary fix to this problem, but Apple needs to fix iOS 4, since this never happenend on any previous software. Let's keep this thead alive, we need this fixed asap!
Oh and let me know if my suggestion works for you too, and especially if your phone reboots too when activating the sensors in the top left after about 10 minutes.