I am running vCenter 2.5 update 4 with 3 ESX servers running 3.5 update 4. All 3 ESX servers are in the same DRS cluster. I have gigabit connectivity between the ESX hosts and the vCenter server.
On my MS Exchange VM, if I create or delete a snapshot the VM becomes unavailable for several seconds and sometimes several minutes. VMware tools are installed and up to date.