This article on ZDNet claims that hyperthreading can *hurt* performance, due to contention in the L1/L2 cache by a second process:
http://news.zdnet.co.uk/0,39020330,39237341,00.htm
Has anyone tested this on Postgres yet? (And based on a recent somewhat caustic thread about performance on this forum, I'm going to avoid speculation, and let those who actually *know* answer! ;-)
Craig