I've wondered in the past about what the best way of going about this would be...especially as we have projects coming up that could be well suited to a separate, small, focussed apps style.

I was just reading the CDBaby interview on O'Reilly and they dropped this small tip:

We're using Subversion's "svn:external" feature to share our model classes ("./app/models") between all four projects.
What ridiculously simple yet clever idea. Thought I'd share!