I dunno why more people don't UML their shit. It's really helpful to figure out what's up before you actually go through all the trouble of doing it. In this case, I'm looking at the structural elements, so I'll just do the relevant class diagram(s), but when I get to some of the features that generalize interaction, I may want to go nuts and do some sequence diagrams to show the behaviour on client/server, client-only modes and why we set it up the way we do.