I'm a french user of Virtualmin GPL.
A few weeks ago, I tried Virtualmin on Debian 5.0 (lenny) and it works very well. (good job ;-)
But it is strange that you remove it from the GradeA support list ( https://www.virtualmin.com/os-support ) Now there is only Debian 4.0 on i386 or amd64.
Is there problems with Lenny ?