Actually, I believe I found the answer, this white paper will tell you everything you every wanted to know about memory in ESX, here's an excerpt....
http://www.vmware.com/pdf/usenix_resource_mgmt.pdf
When the experiment is started, all five VMs boot concurrently.
Windows zeroes the contents of all pages in
physical memory while booting. This causes the system
to become overcommitted almost immediately, as
each VM accesses all of its memory. Since theWindows
balloon drivers are not started until late in the boot sequence,
ESX Server is forced to start paging to disk. Fortunately,
the share before swap optimization described
in Section 4.3 is very effective: 325 MB of zero pages
are reclaimed via page sharing, while only 35 MB of
non-zero data is actually written to disk.7 As a result of
sharing, the aggregate allocation to all VMs approaches
1200 MB, exceeding the total amount of machine memory.
Soon after booting, the VMs start executing their
application benchmarks, the amount of shared memory
drops rapidly, and ESX Server compensates by using
ballooning to reclaim memory. Page sharing continues
to exploit sharing opportunities over the run, saving approximately
200 MB.
