Depends what your goal is, if you want to run a cluster with shared memory between systems, you typically need a very high speed interconnect and very low latencies, your not likely to achieve this within VMs.
Your original message just talked about sharing ram between VMs, you didn't mention why you were specifically interested in doing that.
nate, which vmware you use? VMware ESXi or VMWare Server or something else?
Whenever possible I'd use ESXi, I happen to use vmware server on the system that is sending my email because it's old and ESXi doesn't support the 3ware 8006-2 SATA card in it. Check the hardware compatibility list, the list is fairly short for some things like SATA, though ESX 4 is due out in the coming week(s) which is supposed to dramatically improve SATA support.
Note that out of the box ESXi(with the free license) doesn't support snapshots or cold migrations(between servers) at least last time I checked the feature comparisons sheet. I'll be installing about 12 new ESXi systems soon. VMware server does not support page sharing or memory ballooning, and does support snapshots. I use vmware server 1.x, probably will go to 2.x in the next couple months.
nate, thanks a lot for this answer. I'll give ESXi a try. (Maybe I'll wait till next version)
p.s. Stable xen, has ballooning. You can say for every VM: memory=1024MB, max-mem=2048MB. And you can tweak the memory "manually", without having to restart the VM and without any other problems.
That's not memory ballooning, at least not in the vmware world, and you can do the same in vmware, though if you decide to drop the amount of physical memory allocated to the guest without having the guest free up that memory you'll go into swap pretty quick, so it's not a practice I do, really ever.
If Xen has a specific balloon driver like vmware and inflates the balloon when you change the memory settings that would be pretty cool, as far as I know vmware does not do that, the balloon only kicks in when host memory is low, not when you decide you want to resize the guest.