Yes, it is sad but true. You cannot install the Virtual Server 2005 Migration Toolkit on an x64 server:
What this means is that you need to have an intermediate 32–bit Windows 2003 installation running Virtual Server in order to convert a physical server to a virtual one. Hopefully x64 server will be soported in the near future.
Here is a post that talks about this: ADS 1.1 and VSMT on 64-bit hosts