http://social.msdn.microsoft.com/Forums/en/netfx64bit/thread/03aa04ec-9938-4c55-b351-5defa02e2a80
IIS 7 does not install an ADSI provider by default. You can enable it however as a Role Service for the IIS Web Server. You need to enable the IIS 6 Metabase Compatiblity role service."
It wasn't until I looked closer at the new Server 2008 64bit box that I saw the IIS6 stuff had not been installed -- I didn't load the box. Then I went back and looked at our other 32bit boxes running IIS7 and they *did* have all the IIS6 stuff installed.
But then we faced the issue of how to determine if this component was installed or not, as our app could be run on a box thay may or may not have it ... and apparently it is needed.
So, a little bit of searching around and I was able to locate this article which seems to have answered that question as well: