WordPress has made the source available for its IOS clients here. If you are interested in learning about various techniques for programming on the iPad and iPhone, it is a good set of code to cruise through and review.
Google has announced the Android 3.0 platform preview and SDK & tools here. Quick links if you just want the tools & important stuff (provided by Google):
I decided to post this link in the Programming section to the Joining the Nook developer program post because I describe all the steps I took to get the proper tools, how they were configured, and other errata required to build the “Hello World” application described in the Nook Color programmers documentation. These directions are specifically catered for Mac OS X users.