Full list from the funnyordie website:

Laz Alonso as Doc
Alexis Bledel as Lady Jaye
Billy Crudup as Zartan
Zach Galifiankais as Snow Job
Tony Hale as Dr. Mindbender
Vinnie Jones as Destro
Joey Kern as Tomax
Joey Kern as Xamot
Chuck Liddell as Gung Ho
Julianne Moore as Scarlett
Henry Rollins as Duke
Alan Tudyk as Shipwreck
Olivia Wilde as The Baroness
and
Sgt. Slaughter as Himself
Also featuring Jamin Fite as Cobra Commander
Frankie Kang as Storm Shadow
Geoff Mann as Buzzer
Andreas Owald as Snake Eyes
Daniel Strange as Torch
Kevin Umbricht as Ripper

Also, LOL at the mouse cursor over GI:JOE headquarters =P