Hyper-V Server 2008 R2 SP1 is just around the corner. One of the most talked about additions to the feature list is "Dynamic Memory".
As the name suggests, this is a feature of Hyper-V R2 SP1 that will allow for memory on virtual machines to be dynamically scaled up and down as workloads dictate. Right now, memory is allocated statically and a Hyper-V virtual machine - and all of the software running inside of it - has to be rebooted if more memory is needed for that VM's software stack.
Sounds like the VMware comment about Hyper-V not having "Over Commit" is about to get resigned to the history books.
Windows 7 and Server 2008 SP1 Beta out July 2010
The beta's for Service Pack 1 for Windows 7 and Server 2008 are out next month: