AI becoming sentient plausibility? In lots of sci-fi, it seems that AI becomes sentient (Terminator, Peter F Hamilton's SI (commonwealth saga), etc.)

However, I'm interested in whether this is actually plausible, whether an AI could actually break free form being controlled by us, and if that is possible, whether there is any research as to about what sort of complexity / processing power an AI would need to be able to do this.
There are already programs that have broken free of our control (Morris worm) so that in itself doesn't imply any great computational demands.

Sentience is ill-defined but is certainly not a pre-requisite for a program to do mischief beyond what its creators intend.

It's difficulty to estimate what sort of processing power is required to support human-like intelligence, since we don't know what the most efficient way to achieve that would be.  If the most processing efficient would be to implement a neural network approaching the number of neurons and interconnects of the human brain processing signals at the same rate, the fastest artificial neural network implementations extant are at least 4-5 orders of magnitude short, is thousands of times less power efficient, and doesn't seem to have a realistic way to scale to the number of interconnects required (see this question)