The Center for Internet Security (CIS) is a non-profit enterprise whose mission is to help organizations reduce the risk of business and e-commerce disruptions resulting from inadequate technical security controls.[/i]
CIS_VM_Benchmark_1.0.pdf - This document is a set of general guidelines for securing virtual machines.
You'll have to fill out a form to download, but there is lots of good info (and lots of other good documents, too.)
CIS also has tools for assessing the security of specific OS' as well: Linux, Windows, etc.