I found this in the large and growing list of downloadable vmware appliances: http://www.vmware.com/vmtn/appliances/directory/316. I haven't run it yet, but it claims to provide a way to mirror running windows boxes over iscsi to vmware disk images, then allows you to boot that image under vmware as a replacement for the server with little downtime.
Has anyone attempted something like this with Centos - or know if software mirrors work over iscsi?