Michael,
At this time there are no plans on incorporating sharing between multiple domains on the same server. We get this request a couple times a month in both our sales and support departments so it should already be on our radar for future considerations.
I'll be glad to submit a feature request on my end and will send this directly to the developers for further consideration.
In my writeup to them I will include the feature request for linking shared resources (Contacts, Calendars, Chat, etc) on multiple SmarterMail domains within the same server, as well as public links for shared resources.