There are times when listening to a book makes more sense than reading one, like when you're on a winding road through the mountains on a bus, or when you're on a long car trip by yourself. Or you're on a plane and have already been reading for four hours while you waited for your delayed flight to get moving.
If you want to get the latest bestsellers without buying physical discs for keeps, try Simply Audiobooks
They have a set-up like Netflix for movies, but on two paths. You either get the physical audio books in the mail, getting a new one when you send the old one back, or you register for a certain number of downloads per month, which you can then listen to on a normal (as in non-Apple) MP3 player or can burn to a Windows Media CD.
The cool thing is, if you want to just try it out, they offer one free download each month. Last month it was The Art of War and this month it's T.S. Elliot's The Wasteland. Sometimes they run trial offers too.
I think eMusic is going to give this company some heavy competition though. They offer a free trial where one book is free, then after that it's $9.99 for any book they offer. Overall, it's a better deal if you're a frequent audiobook listener and there's nothing to ship.
Here's another option for older classics: your local library. I don't know about yours, but mine has a ton of audiobooks on CD and thanks to them I learned some Spanish with Pimsleur without spending a few hundred bucks to buy the tapes and CDs. Mine also has free audio downloads of some books. Again, you can't play them on an iPod, but you can play them on a lot of other devices including cheapo MP3 players.