We've had several internal discussions about how we should allow our customers to extend and customize their data.  We considered a lot of different options and thought about a lot of the tradeoffs that need to be made for each approach.  It's because of this that I'm very interested in people's opinions, and as such would like to direct you to Jeremy Miller's "How do you extend and customize a database" post.  Please go and drop him a comment describing what approach has worked best for you so that we can all learn from your experiences! :)