Calendaring in mixed Exchange environment

I have an Exchange environment that involves multiple Exchange
organizations as well as multiple versions (5.5, 2000, 2003). When
sending meeting requests and other Calendar items between organizations
these items do not show up with an Accept/Deny/Tentative button, and
therefore have to be manually added to a user's calendar. Of course,
if the request stays in the same Exchange org these functions are
available.
Is there a way to have Calendar items (with Accept or Deny
functionality) pass between multiple Exchange orgs with multiple
Exchange versions?

Daniel Barry