Season one is out, it was released around May (I remember watching GoT instead of studying for my final exams). Season two will be out during the spring of 2012. And obviously season three is dependent on the success of season 2.
well there are going to be 7 books, 5 have already been published. so assuming it continues to be successful and they don't get budget cuts, i assume there will be 7 seasons. though to be fair it takes George R.R. Martin like 5 years to write a book. so the 6th one will be done by the time the show gets to that point but the 7th one won't likely be written yet. so i don't know what they'll do in that likely scenario.