Last modified: 2011-03-13 18:06:21 UTC
Well, the summary says it all. It could be any arbitrary header, just like: X-MediaWiki-Revision: xxxx It should be fairly easy to implement, and I can't see any disadvantage, but rather some basic support for non-webbased clients - at least while there is no webapi.
If you want that, use the established API (Special:Export).
Well, Special:Export doesn't support fetching specific revisions (action=raw&oldid=xxxx does), and it doesn't signal HTTP 3xx redirects... anyway I forgot about Special:Export, so I sent another feature req.