The quotation marks you see here are because this isn't the first time that George R.R. Martin has "joined" twitter. There are literal dozens of parody accounts and false quotes attributed to everyone's favorite character-killer, and none of them have proven to be real so far. So, until that "verified" button shows up next to George's name up there, don't get too excited. That being said, that does look an awful lot like something Martin would tweet, eh?
UPDATE: Martin's publisher, Random House, confirms that the account is indeed real. Here's to Martin having a new distraction to keep him from finishing his books!