Just to add to this thread for anyone subscribed:
Debian Sarge (3.1) on i386 architecture is officially supported by the Virtualmin Professional install.sh. x86_64 arch packages are building now and will be available in the next day or so, which will also be announced on the news sections of the site.
Ubuntu is also being tested today and I expect it to be available tomorrow or by the weekend at the latest. Another news post will announce that support.