Hey, I'm the designer of Hack Later. The font we went with is Avenir, a font natively available on iOS. Other apps we intend to release under the BrushedType name will also use this font. We're both quite fond of it, and it's nice to stray from Helvetica once in a while.
Avenir is nice. I need to look more into how to use fonts other than Helvetica with Text Kit automatic font resizing. For Pivit that was one of the must-have features--the ability for those who like larger/smaller fonts to be able to change them system-wide. I just recently learned that this is also available with custom fonts.