Do you mean like HyperV's dynamic memory? No, it doesn't do that and I think you need one of the higher editions to support hot add memory (This off the top of my head). That said, VMWares memory management is extremely good and while I don't like it, they do officially support and recommend memory overcommitment so that may be able to do what you need. You can thin provision VDisks in ESXi, though.
(NB: Some / all of this may have changed in the new VSphere 5.1, I'm yet to look properly)
By the way, you can run up ESXi in VMWare Workstation fairly easily (Free trial available). Then you just need the free VSphere Client installed - connect to your new virtual ESXi box and you'll see all the features. The ESXi install is ridiculously simple.
It IS very cut down compared to full on vSphere, but most features require your hosts to be in a VMWare cluster and ideally have shared storage etc.
I am installing 2012 Hyper-V tomorrow on a test rig (medium spec's new server), I shall report back once I have had a few days play. I have only been using Hyper-V for three month and I really miss Vsphere, its just doesn't cut the mustard IMO.
I'm getting ready to test the new hyper-v too. I was hoping to get to it today but it's not going to happen.