I wrote a post a while back detailing a few ways to correct “stuck” snapshots – delta files that existed on the datastore but were not reported by the GUI.
vSphere 5 has a new method of removing them straight from the GUI.
http://blogs.vmware.com/vsphere/2011/08/consolidate-snapshots.html
Note that managing a 4.x host from vCenter5 doesn’t get you the new consolidate option.
