Other than being a new Rails-based site developed during the Summer of Rails, Graffletopia is a great resource for stencils for OmniGraffle, the OS X diagramming and charting app. One of the newest stencils on there is a "Ruby DB modelling" stencil that should help you lay out your models in a neat, easy to edit, way. The author notes:

This is a semi-complete version of a Ruby on Rails way to model a database. The table objects include a space to enter in the declarations that you are considering placing in your model files. I have included a list of reminders of the things that would go into the model declarations towards the right. Each table has anchor points on the left and right of each line and can easily be expanded to include more fields vertically or horizontally.