Jason B, talks about open snapshots:
Know thy open snapshots
VMware snapshotting is a wonderful and powerful technology that affords IT and Developer staff great flexibility and recovery options with virtual machines (VMs) that weren’t so flexible with physical machines or flat out did not exist. With this technology comes the responsibility of using it properly and knowing its limitations. Snapshots have a shelf life that varies somewhere between the moment the snapshot was created and infinity. Boy, that was real helpful wasn’t it?
Jason go’s into some detail of how open snaps work, and how much storage they can take, as well introduces a couple of third party tools to use with ESX.