VMWare Fusion - Reusing Core Ubuntu Install

When testing out new software, Monodevelop this week, I want to keep my daily Ubuntu VMWare guest clean until I decide if I'll use the new tool or not so I use a separate install. I don't want to keep so many Ubuntu installs around so cloning/disk reuse is interesting. I found a good article explaining how to build a core Ubuntu OS and then extend it. Basically, you create a core OS install guest, then share its disk with other testing guests after taking a snapshot in the other testing guest. Taking a snapshot keeps the core disk unwritable.... nice.