My phone (updated, baseband=I7500XXIG8) also reboots in average once per day. It always happens when I'm not using it - I hear the boot sound from my pocket.
It doesn't seem to be connected with wifi, gps, 2g or 3g.
Just now it rebooted, and came to life with more battery than before. This particularly drives me nuts, cause while I find it reasonable to believe that this rebooting can be fixed with firmware update, the battery behavior is simply scaring me. The battery also shows wrong temperature (currently 2.3 degrees Celsius, room temperature is 25 degrees). Ok, I read somewhere that this is due to wrong Fahrenheit to Celsius conversion (which is, well, stupid), but for Li-Ion batteries temperature management is important. I just hope that this stupidity happens only in gui and that there is no freaky temperature management code in kernel...
Back to reboots.
I thought that it has something to do with apps that I installed. So far I uninstalled Steel, Obscura, Useful Switches, but it did not help.
After last reboot I uninstalled also Astrid, will see what happens.
Basically I uninstalled all apps which had - or at least I thought they might have - deeper influence on the phone, registering handlers for various intents. The only "deep" application I still have is Calling Card (intercepts outgoing calls, I need it for calling abroad). Reboot never happened during call, I said already that it always reboots in my pocket. No difference between trousers and shirt pocket, I'm getting paranoid...