Is this consistent across many gas stations? If you have been using only one, try others. It could be their pumps.
I've had this problem sporadically with certain stations and had luck either pulling the pump out somewhat or re-seating it. On at least one occassion, though, there was nothing I could do to make the 'lock' engage on the pump, I had to hold the trigger down with my hand the whole time.
There is a *low* chance you have a problem with the fuel vapor system. However if that were the case, I would think you'd be seeing check engine lights (P0452 & P0453 most likely).