I think this will put a certain amount of pressure on him to move things along. If we were to assume that the TV show continues to the end, and that there are only two more books in the series, the air dates for the TV shows would go like this:
Clash of Kings: April 2012
Storm of Swords 1: 2013 (I think they've more or less confirmed Storm of Swords will run over 2 seasons)
Storm of Swords 2: 2014
Feast/Dance 1: 2015 (I've combined Feast for Crows & Dance with Dragons since much of the timelines are concurrent)
Feast/Dance 2: 2016
Winds of Winter: 2017
Final book (Dream of Spring?): 2018
So, if GRRM doesn't want the TV show to get ahead of the books, he has to produce two books before the spring of 2018 (assuming each of the final two books gets one TV season). How do we rate his chances on this? (No, don't answer!)
Of course, this is really all just for fun and speculation because there are no guarantees that they will continue to make the show or how much they will combine books (hell, Feast & Dance could be condensed into one season!)
I'm not going to knock GRRM for taking whatever length of time he needs to write his books, but to be honest, I think I would be a little disappointed if I had to find out the end from a TV show, rather than reading it in the book.