Hi Luis,
Versions prior to CUCM 8.0(2) are not supported in a VMWare environment. You will need to get to version 8.0(2) before you consider moving to ESXi. In regards to having a physical and virtual CUCM server in the same cluster (ex. pub/sub) that is supported as long as you install the virtual server on a Cisco approved platform (ex. UCS, approved IBM/HP servers).
HTH,
Chris