I must be one of the very unfortunate ones as I couldn't find anything related to this issue on the Galaxy S, only other Android 2.1 devices. But very peeved that I've had to revert to my ooooold Samsung E900 while its in for repair. (missing my superAMOLED )
My phone, yesterday morning just kneeled over and said "Not Registered on Network" when trying to make an outbound call or text and wouldn't accept incoming calls. It was displaying the Operator fine on the lock screen but a cross where you would normally see your signal. I confirmed with my operator that the Sim was reporting in and everything looking OK. Tried a different operator sim and same thing occurred.
Trying absolutely everything that was suggested- resetting, sim cleaning, factory defaults, leaving battery out for 10 minutes, switching to 2g network, driving to a different location to see if it'd pick up, I gave up.
Being optimistic however, I thought to leave it overnight for the magical glitch fairy to appear and correct it, to my disappointment it was the same story in the morning. So I called Samsung Tech Support, and to my amazement I didn't get fed the 1st line tech support bull that I'm used to from big companies like Samsung, only told to take the phone to my local repair centre.
My thoughts is that its a firmware issue (which was IF3 before you ask - value the warranty too much really to be flashing it just yet). However I'll be interested on the outcome and whether it was a hardware error or whether they up the firmware. The reason for the thread however is to ask if any other Galaxy S owners (or variants) had the same issue?