When vSphere was still in beta one of the requirements for using FT was to have hyperthreading disabled. For most people this wasn’t an issue as traditional hyperthreading usually did not improve performance and thus was disabled by default. However with the Nehalem all this changed. Of course I can’t guarantee a specific percentage of performance increase but increases of up to 20% have been reported which is the primary reason for having HT enabled on any Nehalem system.
As you can imagine the HT requirement for FT has been floating around ever since and is a myth which have never been debunked. I’ve spoken with product management about it and they confirmed it’s an obsolete requirement. Hyperthreading does not have to be disabled for FT to work. Or to put it even more strongly: FT is supported on systems which have hyperthreading enabled. Product Management promised me that a KB article will be created to debunk this myth or an entry will be added to the FT FAQ KB article soon.
UPDATE: The FT FAQ KB Article has been updated and includes the following statement.
Does Fault Tolerance support Intel Hyper-Threading Technology?
Yes, Fault Tolerance does support Intel Hyper-Technology on systems that have it enabled. Enabling or disabling Hyper-Threading has no impact on Fault Tolerance.