Just in case anyone else gets bitten, I noticed that a bunch of my dual core opteron machines were losing time sync pretty fast (gaining a few days every few days...heh). I searched back on the list and found some messages about passing a clock parameter in grub.conf. The following worked for me:

clock=pmtmr

These are systems using the Supermicro H8SLL-i boards and Opteron 175-185 cpus and running CentOS 4.3.

Cheers,