I've mentioned this program a couple of times, and now on the Windows Server Catalog site we have a page which shows which vendors are testing their Virtualization products so they are qualified as just another "machine" on which Windows runs... As I find I keep saying we shouldn't care much if the machine is physical or virtual. We already have a KB article which lists the vendors, that we have a joint support arrangement with. And its short, the validation programme's page lists 4 more vendors we're working to qualify. There are a couple missing, but I guess they don't feel the need to work with us to verify Windows on their platform.
On the Hyper-V side we're talking to other operating system Vendors (e.g. Red Hat) about the steps of getting them on the supported list for Hyper-V. Sean has a great post showing different (unsupported) Linuxes running nicely on HyperV. Not supported - i.e. no joint support calls for cross vendor problems, but working none-the-less.