Fortunately, there is a group that is indeed building a replacement. 

I've added this update to my original blog post (since it has been linked to by several other sites), and I wanted to make sure you saw this here also. 

The final word was finally passed down from the group who are building a new tool to replace the Admin Pack for running on Vista, and I have received their permission to repeat it here.  Here is the verbatim quote:

“The new feature with codename “Remote Server Administration Tools” (RSAT – Client) for managing WS08 from Vista SP1 Business, Enterprise and Ultimate will be released as an OOB [Out of Band] component shortly after Vista SP1 RTM.  In the meantime, you can leverage the Admin Pack for managing WS03 servers remotely (Instructions Here: http://support.microsoft.com/kb/930056).  The old Admin Pack will not work against WS08, though.”

So.. that's good news!  You can use the Admin Pack to manage WS03 servers, or you can use the new Remote Server Administration Tools (which I understand will be much like the ones added to "Longhorn" Beta 3) when they become available, on Vista SP1.