Rockstar are teaming up with West Coast Customs (they do MTV's Pimp My Ride) to create a "one-of-a-kind production" of a Banshee Twin Turbo GT. Rockstar say to stay tuned for your chance to nick it - presumably this will be some kind of competition, and Rockstar aren't actually advocating we all head off to the West Coast and attempt to steal it. That would be silly and slightly illegal, so don't do that.

That's pretty much all that has been said for now, but it's worth noting that the Banshee (at least since GTA IV) has been based off the Dodge Viper, so we can assume it will be modified from that (screenshot and some renders).