VMware can do it by live migrating the vm, you will incur a short pause though, and the networking is a bit tricky to setup.. This of course doesn't happen during an unexpected downtime, it's a cold boot on another node in that case.
I have never seen this go smoothly on a production server; it's always WAY slower than expected (if you use any significant amount of memory) and something always gets f'd up wrt the network connectivity, broken caches, etc.
