User blog comment:Werthead/Official: producers will not wait for Martin to finish the books/@comment-5014364-20130329174340/@comment-26308-20130331142555

Unless the series expands to 8 volumes, which GRRM has not ruled out and, to me, may be more likley given that GRRM did not finish ADWD where he had planned, and a substantial amount of material was held back to Book 6.

As for the half thing, I think their rational is that they are about to start work on Season 4 this year, and in a few months they'll have passed (in production terms, though we won't see it for another year) the halfway point of the series, assuming it's going for 7 or 8 seasons.