Microsoft advises not installing beta software, including Windows Defender,
on production equipment. That said, I've experimented briefly with quiet
unattended installs and uninstalls, and they seem to work just fine, at
least on my own local machine--I've not tried this via group policy.
You should be aware that there is no facility to CONFIGURE Windows Defender
via group policy at this time, and Microsoft has posted that even when
released, policy settings will be primarily diagnostic, and not designed for
full administration.
If you want a fully deployable and controllable antispyware application, you
should be looking into Microsoft Client Protection:
http://www.microsoft.com/windowsserversystem/solutions/security/clientprotection/default.mspx