First of all, it's highly unlikely that major movie stars could be signed to do a tv series (even if it was HBO or Showtime). But, assuming that it was possible:
-Would Danny DeVito be too old for Tyrion? Even with makeup? Barring him, perhaps Jeremy Piven (P.C.U.) could be made to look short enough. The midget from Death to Smoochy would also work very well.
-Nicole Kidman as Cercei Lannister. No offense to Ms. Kidman, but I think she'd play a better "stuck-up slut" than any of the previously mentioned actresses.
-Gary Busey Jr. (Tomcats, the creepy religious fanatic from Contact) as Jaime Lannister.
-Sam Neill (Jurassic Park, Merlin miniseries) as Eddard Stark.
-I'd put Russell Crowe as Robert Baratheon rather than Eddard. Robert requires the physique more than Eddard would (and Crowe's supposed to be kinda fat now anyways, which works for latter day Robert).
-Jason Isaacs (The Patriot, Lucius Malfoy in Harry Potter) as King Stannis.
-The Rock would work as Khal Drogo, no really, he would. The Khal doesn't speak all that much, and The Rock definitely looks the part.
-Ewan MacGregor as Theon Greyjoy
-And nobody's tried to cast Jon Snow yet! I'd probably pick Tom Welling from Smallville.
I can't figure out who I'd want for Danaerys though. You'd need someone who was beautiful, somewhat exotic-looking, and very young-looking. And over 18 if you wanted to be true to the books...