What can I say to that other than the currently supported version of Ubuntu is 6.06 LTS and will be 6.06.1 LTS for Zimbra 5. Details of supported operating systems is in the
Product Portal and the main web site download page. So the naswer to your question is, it's not in the near term road plan. The aim is to support Zimbra on a platform that doesn't need updating regularly hence the requirement for LTS version of Ubuntu.
Do I understand you're running this in a Virtual Machine? You'd find performance would be better on real hardware with mail store on a RAID10 array and /opt on something other than the o/s drive. Is this an Open Source or Network Edition install?