Some of the discussion items I have seen about VM's failing under upgrades brings up a worrisome point, for me at least. I look at a VM as a 'forever machine'. A way in which I can set up an environment with any old OS and software and be able to boot and interact with it now, or next year or 100 years from now. It is the answer to 'how do we guarantee access to our digitized cultural heritage?' That puts a great burden on the team doing this work. It means you have to guarantee that an archivist in 2110 can take a VM created today and boot it up and run it on their hyper-quantum-bio computer (the one that you could mistake for a real penguin) under Linux version 15.4.32... and still be able to run a Mathematica file from 2005. I don't know about the rest of you, but I think this is a *very* important part of what should be the spec for VM handling, perhaps one of *the* most important ones.