Also, has anybody out there ever tried to run VxWorks in a Xen VM? I presume there would be lots of difficulties - I suspect network support will be the first. VxWorks is a very interesting little OS, it is used for instance to run the Mars Exploration Rovers 'Spirit' and 'Opportunity', the BMW Idrive system, the Apache Longbow helicopter and Boeing airliners.
Update: Ubuntu seems to be fine security-wise. None of the exploits I attempted gained root or, for that matter, even a user account on the server. But there are loads of them in Metasploit. To test them all would take an eternity.