this sounds too serious to waste any time.
You should immediately open a call with VMware!
I have already raised a call with VMWare. It's a very serious issue am facing.
I believe once the datastore runs out of disk space, it starts deleting VMs that are powered off thus creating free disk space.
Looks like some bug in VMware ESXi 4, since then I have upgraded all my VM host servers to ESXi 4.1 and touch wood haven't faced the same situation again, now I also have vCenter Server thus all the reporting, alerts, warnings are handled by the vCenter Server, give me flexibility and also security.