In the meantime while shared mailboxes are being worked on-you can at least get an rss of another user's inbox (or other folder):
Make a new folder-check the box to make it a RSS/ATOM folder.
https:// INSERTUSERNAMEHERE:INSERTPASSWORDHERE@server.yourd omain.com/service/user/INSERTUSERNAMEHERE/inbox.rss
or .atom
The second INSERTUSERNAMEHERE can be replaced by just the ~ symbol if you want.
If it doesn't work try http:// instead of https:// (some administrators-for whatever reason, don't enable HTTPS)
If you mess up the url-delete and make a new rss folder-they're working on the ability to modify a url after you make the folder-instead of having to make a new one:
Bug 6283 - RSS folder does not display RSS feed URL
RFE to watch:
Bug 9396 - Delegated (shared) Mailbox/Mail Folders in web client
The original:
Bug 6561 - Shared mail folders