I have recompiled the model viewer for XNA 3.0.
This build should also support 64bit OS's, though I haven't been able to test it.
I have made no changes to the viewer other than migrating it to XNA 3.0.
When I get some more time for this project I will update it based on the feedback I have received (thanks all).
The old version for XNA 2.0 does not support 64bit OS's, it is a very small change to fix that, but I'll leave it as is for now. Feel free to contact me if you need the XNA 2.0 version with 64bit support.
As always, feedback and bug reports are most welcome - You can use the feedback form in the viewer, post here or in the thread at Creators Club (http://forums.xna.com/forums/t/14018.aspx).