Microsoft has RTW'ed their planning tool for OCS 2007. I have seen and tested some of the beta's and it ended up being a neat little tool.
You will input parameters such as #users, regional distribution, demand for high availability features and end up with a topology drawing looking like this -
One of the good features are all the links to resources like planning steps, hardware configurations, bandwidth capacity etc.
It's a very good start, but there are still room for improvement like the ability to export the drawings in Visio format and e.g. removing the requirement for high availability at the small sites.
If you want to do more detailed capacity planning and sizing the you should definitely take a look at the OCS 2007 resource kit - it contains very good chapters on this and Excel spreadsheets for doing your own sizing.
Find the tool here.