Based on the excellent feedback from the representatives, we ended up with two significant changes in our approach. The first change is simply moving the URI of the original resource (URI-R) from the Alternates: response header to a separate Link: header. The information returned from the TimeGate (URI-G) and Memento (URI-M) is the same, it has just moved from one header to another.
The second change represents a larger change from the previous model. Instead of URI-R redirecting (302 response code) to URI-G when it sees an Accept-Datetime header, URI-R always returns one or more Link: response headers pointing to one or more TimeGates (whether or not the client sent an Accept-Datetime header). It is then up to the requesting client to either use the URI-G value(s) returned by URI-R or to use their own value of URI-G. This should be easier to implement for servers since they just have to send a Link header, and it should also work better with existing http caches.
We're in the process of changing over our demo systems. Herbert has updated the slides to reflect these changes and integrated them all into a single presentation:
Memento: Updated Technical Details (February 2010)
The original slides from November 2009 are still available. Thanks to Kris Carpenter (IA) for inviting us out and setting up the meeting.