It's sort of hard to catch, but apparently, former president George W. Bush made a sort-of appearance in season one of Game of Thrones...his head did, anyway. According to the DVD commentary for episode ten of the season, in the scene where King Joffrey forces Sansa Stark to look at the decapitated heads of her father and former handmaiden upon pikes on a castle wall, the head just to the left of theirs is of Bush.
Creators David Benioff and D.B. Weiss explain in the commentary:
"The last head on the left is George Bush. George Bush's head appears in a couple of beheading scenes. It's not a choice, it's not a political statement. We just had to use whatever head we had around."
You can check out the clip of the scene (sans commentary) above. Does this make him technically a guest star?