The forums at westeros.org
are probably the definitive boards for the show. Each episode has threads for "No spoilers", "Book spoilers", and "Book + TV spoilers". Presumably if you've read the books, the Book spoilers boards are the ones that won't reveal anything you don't already know, and the Book + TV spoilers has show-specific stuff leaked from scripts or cast interviews. They also have a very specific thread dedicated to the differences between the books and the show.
Personally, I'm comfortable over here and would rather keep this AVS as my board of choice for discussing this show. Unless I'm instructed otherwise by an AVS moderator, here's the completely arbitrary rules that I'm going to follow:
(1) I'll not reveal anything specific about chapters that are ahead of where the show is.
(2)I'll occasionally bring up things that I think are interesting differences between scenes in the show and the books, but not in a nitpicky way.
(3) I WON'T talk about "missing" scenes from the book that aren't in the show, because they might have just been shuffled around and we could see those scenes next week.
(4) I'll occasionally talk in extremely broad generalizations about future stuff in the books (i.e., I've mentioned several times already that the scale of the story gets HUGE).
The only place where this might cause some problem is with #2 if things have been shuffled around in the show. For instance, there is a lot more backstory in the book given to the conversation between Ned and Robert in the crypt. Someone here asked a question about it, and I was intentionally vague in answering because we'll probably get most of that backstory in the show eventually and I didn't want to spoil it.
If you've got a problem with that and you're not a moderator, you might as well set me to "ignore" now.