I have in the past virtualized an Oracle ERP database successfully on VMware and hyperthreading was turned on. In my experience, there was no problem and we didn't see any negative performance. The database was moved from a unix OS to RHEL. My experience also indicated some performance factors that were affected due to CPU reservations and limits if set.
Note that the virtualization was done on vSphere 4.2 and not the latest vSphere 5.1. I can run a test in my lab and come back to you with the test results. If you have a test environment already you can simulate the test as well and find out if hyperthreading is affecting or helping with the latest versions.