It is a high quality and popular vector drawing app (4+ stars on app store) and the code is clean and simple to understand. I am not sure you will learn anything by just reading through it if you're not building a similar kind of app though.
Quick googling also yielded these lists:
http://maniacdev.com/2010/06/35-open-source-iphone-app-store...
http://en.wikipedia.org/wiki/List_of_free_and_open-source_iO...
How to architect your app really depends on what kind of app you're building.