My wifes has started to do the same, the iphone 3Gs was mine, I updated it to ios 5.0.1 while still using it and it was fine. I did a backup restore of her old 3G phone and again it was find for awhile.
We noticed she doesnt have any data part of her package so we have switched off the data, since then we have this issue.
I had to install a new docking part of the phone as her mic was faulty, I did the sim out, switch on 3G and data and then sim back in. Switched the phone off and then back on. all good.
I stripped the phone down adn replaced the part. again all good. That I night my wife said Shall she turn the phone off in which I replied it should be fine. When she turned the phone on in the morning, the reboot problem was back.
I'm wondering if the problem is to do with the Data on and off. I've also noticed that when the phone is up beofre it reboots, you can pop out the sim and the signal stays the same.
When it does have a problem, she now does a quick reset from the phone which takes about 5 minutes and shes back up and running. its big pain!
Think I will get her on a data package and then try again