One of my favorite bloggers and performance guru Scott Drummonds has posted some info on Hyper-Threading and vSphere.

On earlier Intel Processors it was best practise to disable Hyper-Threading. But with the I7/Nehalem/x5xx processors from Intel you should be adviced to ENABLE it. In many use cases it can give a 10-30% performance boost. Check out Scotts blog for all the goodies.