Fact: HBO generally prefers unknowns for their series.
Really, specualtion about casting is waaaaaaaay too early. HBO has yet to give the show a green light even, they only have the rights. It will be quite a while before any plans for the actual show are set in stone at all.
Jedisage, I'm totally with you, ASOIAF is not possible without the ambiguity of the character's tendencies toward good and evil. It is one of the reasons Martin's epic is a page turner. Unlike something like Harry Potter (I am not a fan of that series AT ALL) where you know what will be happening with some givens like "harry will live" Martin keeps you totally guessing because his story is based on how people actually interact and quarrel. Very few characters are "safe" in his world (ahem to the Starks...lol) whereas other storys put in a core group of main characters early on and you can trust they will be there throughout the story. I don't know about everyone else, but that idea has tired me out because it just puts a drastic sense of fiction in stories. I know fantasy isn't supposed to be realistic all the time, but come on, you know? You lose some of the thrill if you know Mr. Potter isn't actually in danger or that the emperor won't lose to Mace Windu no matter what it looks like should happen. The ambiguity is a great thing in ASOIAF and it won't change through the rest of the series.