Sometime last year I found out that Apple was using the fleegix.date.Date plugin to build the timezone support in the MobileMe calendar, and recently had the opportunity to take a look and see what they did with it.
The MobileMe calendar is quite pretty, as you might expect with an app written with SproutCore. It still has some pretty rough edges (I'd say even lagging behind the UI we built for for Cosmo (AKA Chandler Server) in a lot of areas), but is mostly speedy and pretty solid.
Seems pretty inefficient to me to push that out to all users (or even just all users who have timezones enabled) with no regard for how much of that data is actually ever used. But of course, MobileMe is still a relatively new product, and has a lot room for optimization.
I'm pretty excited to see a company like Apple building a major end-user-facing product using a Fleegix.js plugin, and even more enthused about the idea that the code is getting some real large-scale usage.