To clarify, we are only looking at instances where you are kicked back to the XMB without getting a prompt for a crash report. System freezes shouldnt be counted with them.
It happened to me 10-15 times after game release... 3-5 during the beta. Usually while flying (CAF mode) but also while in a tank. All sorts of different scenarios on most maps. I own a PS3 slim 320GB. Hope this helps.