http://news.zdnet.co.uk/0,39020330,39237341,00.htm
http://www.xecode.com/blog/2005/11/20051121_001281.html

Hyperthreading allows different elements of a processor to run different code at the same time, which Intel has claimed will boost chip performance and allow a CPU to process nearly twice as much information as one without hyperthreading.

“Our customers observed very interesting behaviour on high-end HT-enabled hardware. They noticed that in some cases when high load is applied SQL Server CPU usage increases significantly but SQL Server performance degrades,” wrote Ocks.

HT makes a hot spot in L1 and L2 caches according to the posting by xecode.com. And, it is already known that HT can make programs vulnerable in a special settings.