This update actually went live a couple weeks ago, but a few bugfix releases later, the official announcement is finally here:

Our free mobile apps just got even better! We’ve added more than 40,000 interactive exercises to our Android, iPhone and iPad apps so you can study anytime, anywhere.

Get the apps now:
Android: http://khan.co/KAAndroidUpdate
iPhone/iPad: http://khan.co/KAiOSUpdate

 - This project also updated the exercise experience for people on mobile browsers, who should now have a much better interface than they used to.  This whole thing was actually a massive internal project that took a bunch of coordination and complex synchronization, so big kudos to the team.

 - Trivia time: this project was internally known as "Exercises on Mobile"...or more fondly as, "Ex-on-Mobile" ....which maybe had me thinking for longer than it should have that an oil company was somehow involved :-p