I too wanted text-to-speech (TTS) capabilities to read e-books. I don't know if this is true on the Mac, but on the PC Adobe Reader v6.0 allows TTS without the Digital Additions add-on. I use that for TTS capabilities with Adobe e-books or other PDF files.
Acrobat 6 and Acrobat 7 both support screen reading utilities on Mac and Windows for documents that have the appropriate permissions ("Content copying for accessibility"), as well as supporting the reading EBX encrypted files (eBooks).
Acrobat 8 supports screen readers but does not support EBX encrypted files.
Digital Editions supports EBX encrypted files, but version 1.0 does not support screen readers.
If you require both eBook support and screen readers your best option at this point is to use Acrobat Reader 7.0.9, until the screen reader functionality is added to Digital Editions.
Check out YAKiToMe! at www.yakitome.com. YAKiToMe! includes multiple free high-quality text to speech voices, much better than the MS voices. You can convert MS Word documents, .txt files, PDF files, RSS feeds, email, and any other electronic documents. This might be a reasonable solution for your e-book. At worst, you can cut and paste into YAKiToMe! to get your document converted. YAKiToMe! produces MP3 files or WAV files that can be easily podcast to your iPod or downloaded to your computer.