Bob,
Data Archiving and Purging for Contracts and Sourcing is a planned feature for the Ariba 9r2 release (scheduled for Q1 2014). The "purging" aspect of the feature will accomplish the eDestruction use case -- purged documents will be completely removed from the Ariba system (with the option to archive that data to a non-Ariba system if necessary).
The use case that you've provided regarding "retention tag/retention date" functionality would be addressed by the feature as well. Configuration of the archive/purge rules will allow businesses to define rules that determine what is archived and when it is archived, allowing for a "sliding window" for the archive/purge process. The functionality would also offer a preview of archive/purge candidates that can be reviewed prior to the archive/purge taking place.
On-Demand does not have an archive/purge feature at this time, although I know it has been discussed in detail and is under consideration for a future release.
We will be providing much more information regarding this feature (and many other features) during the 9r2 feature discussions that will be occurring in early 2013.