I'm considering running Exchange Server on a hyperthreading-enabled system. Does a hyperthreaded CPU truly provide the same performance as two real CPUs?

Intel would certainly like you to think so, but in reality a processor with hyperthreading enabled gives you 125 to 150 percent the performance of an equivalent nonhyperthreaded processor. (The Microsoft article "CPU and Memory Scalability for Exchange 2000 and Exchange 2003" at http://support.microsoft.com/?kbid=827281 cites the performance increase as 25 percent, but in my experience it's typically a bit higher.) The actual performance increase will vary depending on the mix of threads running on your Exchange server, but in general, one hyperthreaded CPU will act like one and one-half real CPUs.