I've seen a bunch of clever rigs for Bajas and bikes.
A few welded up a rear metal holder that mounts in the tow hitch receiver and sits level with the tailgate, allowing for a bike with a wheelbase longer than the bed.
I've moved a Honda Rebel for a friend by simply using a large and sturdy piece of wood to do the same (I actually extend the wood through the cargo port and I tie it down at the front using straps through the cargo hooks - then roll the bike up a ramp and onto the wood and tie it down front and rear to the hooks.
I figure if I'm ever pressed I probably COULD move one of our Moto Guzzis or our Harley (Sporty) that way too, but we'd be pushing the limits.
One really cool rig I saw had a cage built on top of the roof to dirt bike could be rolled up with the front tires on the roof and the rear tires sitting in the bed.
I've got pics of all this stuff, but I'm too lazy to find a pic host to post em. I can email if you want.