I could actually add to that, now that I have the numbers in front of me. On overall advance poll voting, it's gone from 5.4% in 1997 up to 10.5% in 2006. Special ballots, which is what you're generally referring to—which can also be by mail, but usually when you go to vote at the returning office you're going through that special ballot process—were 1% in 1997, 1.5% in 2000, 1.8% in 2004, and 3% in 2006.
There you see that in the space of four elections it tripled; it's still a small percentage overall, but it has tripled as an option.