Not particularly. Like, maybe twice a year.
So if it happened to you once, I wouldn't worry about it. But if it's happening a lot you should try restoring from a backup and, if that doesn't' work, going to an Apple store.
I've had it happen periodically, even on the 4S, however never when jailbroken.... go figure, only on stock iOS. It's not like everyday, but each of my iPhones has had freezing out of nowhere and have to hard reboot. Usually when using Safari, but other times I'm not sure what caused it... almost like swiping and clicking an app too fast or sometimes after waking up and sliding the locker.
its kinda crazy cause mine never locks up, are your phone GSM or CDMA?
just wondering if maybe that has anything to do with it?3
who knows, now i'm just fishing...