Basecamp Mobile is written in CoffeeScript using our in-house Cinco mobile framework, which ties together Backbone.js, Zepto, the Eco templating language (https://github.com/sstephenson/eco), and Stitch (https://github.com/sstephenson/stitch).
I'm presuming that the back-end for Basecamp Mobile is the current Basecamp Rails app -- I'm curious about why you decided to use Stitch to package your JS, instead of what Rails is using (probably Sprockets or Sprockets 2).
There are a couple of reasons we went with the Stitch approach. For one, we found the CommonJS module system (as implemented by Node.js) to be an elegant way to declare dependencies and share state between closures.
Second, our test suite runs headlessly in Node.js. By using Node's require system, we're able to get useful information and correct filenames in backtraces during development.
Any chance Cinco will be up on GitHub in the future, or are you guys holding it close to your chest?