I've only peripherally been paying attention to this issue that came about when Amazon released the new Kindle 2 with a "read aloud" text-to-speech feature. I read Neil Gaiman's and Wil Wheaton's blogs and Twitters about the subject and think I am probably in agreement with them; that this is not the same as an "audio book" read with feeling by a real person, but rather an electronic rendering much like the computer on Star Trek or your GPS. No feeling, no sense of timing. Just words pronounced by a synthesizer. Not something most of us would want to listen to for hours on end.
This seems to be a great feature for the visually-impaired, but I'm not sure ANYONE would want to listen to a synthesized voice for the length of a typical novel. Perhaps a newspaper or magazine article (which Kindle 2 allows you to download), but not a novel.
Still, from my limited point of view, it seems more than fair of Amazon to allow the author and/or publisher to decide whether they want to block or enable the text-to-speech feature on their own works.