Setup - 5 x 3.5 ESX Servers and Virtual Center, VC 2.5.0 (build 64192)
If I check the NTP settings in the time config gui (as below), without changing anything it screws the time up on the ESX box
Go to Configuration tab in VC, "Time Configuration", "Poperties", "options", "NTP Settings", then click "ok", then "ok" again.
If I then run "ntpdate -q 10.x.x.x" on the ESX box, against the time server I can see it has immediately lost or gained anywhere up to 20 seconds. (I know its the GUI actions doing this, as I run the same command immediately before doing it and everything is fine)
Has anyone else noticed this, if the loss or gain is less than about 15 seconds it does synch itself again, any more and I have to a manual restart of ntpd.
maybe someone with a test environment could spare a minute to try the same thing. 
cheers