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.