Audio books continue to grow in sales year after year. With the massive sales of iPods, MP3 players, and MP3-enabled cell phones, not to mention the rapid adoption of all-thing-tech by the younger generation (who I'd guess would rather be listening to their iPod than cracking a book), these sales are sure to grow.

According to a BusinessWeek article from April "In 2005, the Audio Publishers Assn. (APA) estimated the size of the
total audiobook market at $871 million, up 4.7% from the previous year.
The APA also released a 2006 study that found almost 25% of the U.S.
population listens to audiobooks." I'm certain that these numbers have only continued to grow in the past year or so, especially as more titles are becoming available every day.
My wife has joined in on the fun by recently launching an audio book ecommerce site with the clever title of AnEarForBooks.com. If you're interested in exploring the world of audio books, I encourage you to download one (or more) and give them a listen. We find them quite enjoyable on long road trips to Tahoe and Yosemite, and also while traveling by plane.