I also returned my phone for a replacement. The 2nd phone still continues to reset but not as frequently. Both my girlfriends and my original phone had the reset problem, her new one doesnt do it at all, of course mine does.
I have probably wiped my phone completely 10 times. Along with running it with zero market apps, none of which helped the reboots. Sometimes in the middle of the afternoon and also shortly after charging.
I also had an issue where the battery would drain crazy fast and none of the reboots/ factory resets would fix it. "Android OS" would go nuts and run all the time on a factory phone. I actually got this to stop after a couple weeks of aggravation. Pull the battery out and leave the phone alone for as long as you can bear, sounds weird but im sure it worked, my process list and battery usage is back to normal.
I think if i want to exchange my phone again ill have to use samsung warranty or something as i bought my phone off Wirefly.com and they only offer exchanges for the first 30 days.