Paul Ellis mentioned the ASND format in a recent post on Inside Sound, an Adobe audio blog:
'The Adobe Sound Document (ASND), which is pronounced “a-sound”, was introduced in Soundbooth CS4 to address the “Oops Factor” in the world of audio editing. When you save your edits to an audio file using the ASND format, effects and volume changes are saved in a non-destructive way so that you can open the file later and tweak these settings. Additionally, the ASND format supports the notion of Snapshots. Similar to Snapshots in Photoshop, this gives you the ability to save multiple versions of your edits and allows you to easily switch between them. By default, Soundbooth will ask if you want to save the original document in the ASND package so you can always retrieve the untouched file you started with.'
Also, I like the New Soundbooth Feature: Match Volume that I'd missed from using Audition. Match Volume matches your files to a specified dB volume or to matches them all to the level of one of the selected files. CS4 Soundbooth features are summarized in this video: