I had that happen once after Enterpise Activating one of our VPs accounts onto a new device (at 4pm on a Friday, naturally). RIM had me go into the Calendar on the device, turn off the wireless sync, wait a few seconds, turn it back on, wait a few seconds, then delete the Desktop [SYNC] service book and then undelete it (causing it to slow sync).
Their other suggestion was the usual "just delete the BES user account and recreate it". Ugh.