See ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-05:09.htt.asc

Briefly, the article claims that there are timing attacks on 
hyperthreading machines that permit disclosure of, for example, 
cryptographic keys.  The advisory I quote does not have sufficient 
details to let me evaluate it definitively, but I would say it's 
extremely plausible, given other results on timing attacks -- see, for 
example, http://crypto.stanford.edu/~dabo/abstracts/ssl-timing.html

		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb