Comment 6 for bug 602625

A similar issue was noted for Urban Terror, which uses the same engine: http://forums.urbanterror.info/topic/20357-bug-x86-64-wont-start/
An ioquake bug for 64bit Intel is mentioned: http://bugzilla.icculus.org/show_bug.cgi?id=4331
Sounds like a significant problem, though only for certain systems.