My AT&T iPhone 4 has only frozen when I have it Jailbroken and I attempt to do too many things at once on it. For instance, from the lock screen, sliding to launch BiteSMS, pressing home to get back to the Springboard, and then swiping down to open the notification center rarely gives me problems but sometimes causes my springboard to crash. Not really a freeze, but an interesting error nonetheless.
My wife's Verizon iPhone 4, however, completely locks up seemingly unprovoked. She won't let me restore as new though. Stubborn. Her co-worker's VZW iPhone 4S does the same thing.