The problem is the author takes years to write the next book, so the TV show could theoretically catch up before the written series is finished.
He doesn't ALWAYS take forever...
1. A Game of Thrones August 1996
2. A Clash of Kings February 1999
3. A Storm of Swords November 2000
4. A Feast for Crows November 2005
5. A Dance with Dragons July 2011
He cranked out Storm of Swords in a year and a half. I think with the TV series as a motivator that he'll get the other ones out before they're set to shoot.
Also, he doesn't work on the chapters in order, or even work on just the chapters in one book. What I've read is that most chapters for 6 and 7 were already written before book 5 was published.
It'll be interesting to see if they recast any of the young characters as the series progresses. It seems like only two or three years have passed in book time... but it'll be 6 or 7 in series time if they're doing multiple seasons per book.