I've had my PS3 "lock up" rather than freeze on many instances. It usually occurs when I place a regular DVD in the PS3 and start to play it. The only fix is to flip the power switch on the back of the PS3...scary move when I remember how many Windows based computers I've seen crash after flipping the switch without shutting down correctly!
It seems that the current theory is that the network is causing problems. I can attest that my PS3 refuses to cooperate with my wifi network...its hit or miss (mostly miss) even though the PS3 states it has an excellent signal. My laptop and Wii have never sputtered with the wifi, even when further away from the router than the PS3 is located. I have finally completed the Cat 5e network in my house after nearly 9 years of dangling unused upstairs. I will be hooking the PS3 up tomorrow to see how the hardwired network works, and to see if the "lock ups" still occur.