Before anything else, I should say that a good friend of mine actually had a fuel fire while fueling his -6A from cans. He was fortunate; it did no damage. He had fire around the filler neck (quickly extinguished by dropping the cap into the fill neck), and a small fire on the ground due to spilling some of the can's contents as he recoiled from the fire. He will no longer fuel his plane except from an airport pump.
With that out of the way...
If I'm not mistaken, commercial pump hoses are bonded from nozzle to pump. Having said that, I suspect that if you fail to attach the ground line to the plane, and hold the nozzle over/in the fill neck without actually touching the neck, you could still get static buildup. I think this because it's my understanding that the fuel's flow through the hose can generate static charge buildup, similar to rubbing your shoes on carpet.
Having said all that, I'm pretty sure that plastic gas cans are impregnated with enough conductive material to bleed off any static charge, as long as they are in contact with ground or the source/destination of the fuel. And if you can believe the Gubmnt, metal is no help. See the last sentence in the 2nd paragraph. Also note the recommended procedure to minimize risk while transferring fuel in either direction.
Last, IIRC, the reason I was told to hook up the ground lead before getting the hose at the fuel farm was because the plane itself has the potential (pardon the pun) to build up a static charge in flight (think static wicks on high/fast planes), and there's a risk it could retain that charge as you approach it with the grounded fuel hose.
I continue to fuel from plastic gas cans; I just try to incorporate whatever new info I learn as time goes by.
edit: Should have added: Touching the can to something metal on the plane before opening either fuel container should dissipate any static charge differences between the fuel and the plane.