Haha! I wish you well in your endeavor. Re-writing my command/undo system for the 3rd time here. I am fairly certain this one will be a keeper aside from tweaks here and there - must say I don't want to have to do this again, again. Learned some valuable things though!

[edit] Curious - how are you going about it?

Eek... documention....

I prefer to save relevant objects in a scene file in memory each time an action if taken. Once you set it up right, you can add new functionality to your program without having to add any new undo code, because you are just saving states. For memory consumption, it's important to only save the changed objects each step.