It was actually mentioned by the show's co-creators in the Season 1 DVD commentary. On Wednesday, Reddit user SidIncognito picked up on it and posted this photo on the site.
So, what is George Bush's decapitated head on a stake doing in "Game of Thrones"? The co-creators said in the commentary that it was just lying around.
“Back up," one of the creators says in the commentary, which you can watch below. "The last head on the left is George Bush. It’s not a choice, not a political statement! We just had to use what heads we had around.”