I am wondering if it is possible with vCenter 4 to setup and Alarm that will monitor the percentage of disk space used inside a VM's/Guest OS. I looked but could not find anything definitive.
I think your best bet would be to rely on third party tools, such as MOM or SIM, especially if you want real time monitoring.
you may want to look at this thread that will give you options with PowerShell
I dont think there is a way to monitor the VMs disk usage from VC. But you can do the same for datastores. I dont think it will be of great use in your case.