The following line will generate (pseudo-randomly) a static MAC address, suitable for use with a KVM virtual machine;
date +%s | md5sum | head -c 6 | sed -e 's/\([0-9A-Fa-f]\{2\}\)/\1:/g' -e 's/\(.*\):$/\1/' | sed -e 's/^/52:54:00:/'
Similar nonsense can be done with Hyper-V and VMware.