I heard this on the 1UP podcast The Oddcast originally, but apparently Jeff demoed Def Jam Rapstar on-stage, giving an amazing rendition of Snoop Dogg's G Thang. Examiner says:
In a short demo, Giant Bomb's Jeff Gerstmann spat Snoop's lines from “G Thang” while Rapstar creative director Jaime King handled Dre's parts. It was at times hilarious, sad, and kinda bad-ass. We'll have some more details for you Tuesday.
But no video. I've looked around the internets, but to no avail. Damn you Examiner! Hopefully the guys will do a quick look when the game hits this Winter. We all know what Jeff and Ryan are capable of when they are combined into a tag team. "Girlfriend" all the way.