Taptrix (the developers behind Inkpad and Brushes) was YC S10, though if I remember correctly they joined YC after their Brushes app was already a smashing success on the iPad. Brushes was released the same day the iPad 1 came out and the two guys behind it were both Apple veterans. [1] [2]
Brushes — This startup created the Brushes application for painting on your iPhone or iPad. The app was used to create the well-known iPhone covers for the New Yorker. With 250,000 paying users and $60,000 in monthly revenue, Brushes apparently holds the YC record for most profitable startup on Demo Day. Its eventual goal is to become the “Adobe of touch devices” by building a suite of apps. [3]
If I had to guess, I'd say the devs probably have enough money to retire. Congrats! Thank you for contributing your code to the world.
[1] http://appadvice.com/appnn/2010/03/brushes-ipad-launch-scree...
[2] http://techcrunch.com/2010/08/24/y-combinator-demo-day-2/
[3] http://venturebeat.com/2010/08/24/five-new-y-combinator-star...