I've just completed the first fully successful test install of Virtualmin on a CentOS 6 i386 system. I haven't done a full test install on x86_64 yet, but will in the next day or two (I don't expect any issues, but until tested, I don't know for sure).
This means that for new deployments of CentOS, RHEL, or Scientific Linux, we recommend you use the latest version of the OS available. Version 6 has much newer components in a number of areas, and it actually makes it possible to use several Install Scripts that cannot be used on version 5. In particular, Ruby on Rails begins to be usable again on version 6, as do several PHP apps that only support 5.2.x.
There are no known issues that I've spotted in my testing, but because it is a new OS, I highly recommend you plan for some delays in deploying any systems on this OS version, both for the sake of problems in Virtualmin's installation that we haven't identified yet, and for unanticipated problems in the OS itself (we found an interesting issue with the CentOS 6 kernel when running in virtual machines, for instance, which cost us a couple of days of troubleshooting when working on Virtualmin installer support).
As always, if you run into any problems, let us know in the ticket tracker.