below are some examples
1st three letter to describe Datacenter location - TEX ( It would be handy if you have multiple datacenters )
next three letter to describe OS version - 2k3 / 2k8
next two / three letter to describe use of this VM like - AD / SQL / DB ..etc
Next three letters for a serail number - 001 , 002 , 003
TEX2K3AD001 .
let me know your thoughts :-)