Actually, TracFone is a "friend" of AT&T's. TracFone does not own any wireless networks - they only subcontract to other companies. Read: My Tracfone and iPhone both run on AT&T's network.
And yes, phone companies are required by law to allow porting anyway, even to their rivals.
That said, I think I considered porting my number from my Tracfone when I first bought the iPhone, and I had some difficulty getting ahold of some sort of passcode number I needed, but my guess is that this could've been figured out if I had persisted.