I've been following iD development on github (https://github.com/systemed/iD/) since last year, and I honestly think that reading through its source is the single best case study I've come across for application development with D3. It's pretty phenomenal. Perhaps I'm a bit biased since I work on map stuff as well, but I think pretty much anyone thinking about using D3 to build anything larger than one-off visualizations would do well to see how it's put to use in iD.
Just added my first changes through iD, I didn't expect something so simple, functional and ergonomic. Little details like contextual ~pie-menus, geometric operation suggestion for instance segment midpoint as a trigger for insertion, very pretty. Did you work on modeling tools prior to that ?
Hats off to you guys, great work!