Probing memory is usually done from the BIOS level ( OpenBootProm on SPARC ). I would be very hesitant to poke the memory on a running system that I wanted to stay running.
If you do find a tool please let us know, it would be very convienient to be able to test this without bringing down ESX ( after moving all the important VMs off ).
In the SPARC world I have never seen a memory error that was not reflected in the "messages" log.