I suppose that I was lucky (or unlucky?) in that I've had my phone since September 2010 and only had this issue happen for the first time early this month (November 2011). So that's over a year with perfect service. No problems at all with my Captivate. At this point, however, it is turning itself off at least once a day and often two or three times. Always during the day, never overnight when it is plugged in to charge. It has become a huge inconvenience, as well as a difficulty for my family who are unable to reliably get in touch with me. Adding to the annoyance factor is that the darn thing takes FOREVER to turn back on. Not normally an issue, since I don't turn it off.
Of course, I am well out of the warranty period, having owned the phone for 14 months now. And it wasn't a cheap phone when I bought it, even as my allowed "upgrade" on the account, it still ran me $200 at the time. Would my best bet be to contact AT&T at this point, or Samsung directly? Or am I simply SOL since I've had the phone for over a year?