OK, I've figured it out.
Group Policy Management Console reads the ADMX (and ADML) files from your c:\Windows\PolicyDefinitions folder, and then builds the policies in the SysVol\\Policies folder to replicate to all DCs and apply to all computers.
Windows 7 includes updated ADMX. To use these ADMX on your domain you can do two things:
1) Install RSAT on a Win7 Domain Member and admin your GP from there.
2) Build a Central Store to hold your ADMX and copy in the Win7 ADMX/L.
The disadvantage of 2 is that the Group Policy Editor is a little newer in Win7 than Server 2008, but you will get all the new Win7 settings.
To build your central store follow these instructions:
Every .admx in the Central Store will override the local .admx.