Have just updated my box to CentOS 5.5 and am pleased to say that I haven’t noticed any issues as yet.
Just in case anyone else is thinking of doing so.
i’ve updated 3 systems and haven’t encountered issues either.
Several physical boxes and Xen hosts with Cloudmin. No issues at all.
Likewise, no issues - Be prepared for a little longer reboot to wrap-up the upgrade. My box took about 9 minutes to come back to life.