I had this problem and ended up bringing it to the genius bar, this is after Apple Support had me do a system restore and the problem persisted over the next week. The genius bar reinstalled the firmware and I'm still having the same issue but it's not every day, now it's restarting once every couple of days and I went from having great battery life to terrible battery life. So I'll be calling Apple support again this week. I'm a big iphone fan but this version of iOS just seems very "buggy" hopefully they can get a handle on it soon. iOS 4 and 5 ran very smooth with no issues at all.
This isn't a common issue, so a common solution is not going to be had.
1.) Try a hard restart.
2.) Try a restore.
3.) Return it to Apple for a replacement.
I actually read another thread from someone else where a number of users were having reboot issues, I too have experienced a few. It happens for me generally when entering or leaving a text message, maybe once every couple of days. Not a big enough deal to worry for myself.
I hear it happens on iphone 4s's as well on 6.0, so it probably is just a software issues affecting a limited number of users.