I think the poster was referring to voice actors. John Kleese (spelling?) from Montey Python made a guest appearance in Jade empire, Michael Ironside is the voice of Sam Fisher from Splinter cell, The aforementioned Samuel L Jackson appearance in GTA, Bam Margera from Tony Hawk Project 8, etc.
The Command & Conquer series manages to get quite a few recognizable faces like George Takei, that guy who played J.J.Jameson from the Spiderman movies, and again Michael Ironside. Personally I think this is great.
I'd love to see more games with "real" actors in them. Most of their performances are miles ahead of the VO cast of other games. (anyone else still cringe at some of the Voice Overs in Fable 1?)