...in a practical sense, only Windows XP and Windows Server 2003 (in all their service pack levels) are vulnerable to the WMF flaw. ...all versions of Windows back to 3.0 have the vulnerability in GDI32. Except for Windows XP and Windows Server 2003, no Windows versions, in their default configuration, have a default association for WMF files, and none of their Paint programs or any other standard programs installed with them can read WMF files...
So the vulnerability is there on all platforms but it seems that only Windows XP and 2003 are easily exploitable. Unfortunately this still means that majority of Windows computers out there are vulnerable right now. And at least Windows 2000 becomes vulnerable if you're using many of the available third party image handling programs to open image files.