If you haven't seen it already there is a post on the OpsMgr product group's blog talking about virtualization of the various OpsMgr roles. If you're considering virtualization you might want to check this out at http://blogs.technet.com/momteam/archive
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Read the complete post at http://blogs.technet.com/monitoringmicrosoft/archive/2007/10/06/some-thoughts-on-virtualization-of-opsmgr-roles.aspx