Chameleon is a drop in replacement for UIKit that runs on Mac OS
X. In many cases, your iOS code doesn’t need to change at all in
order to run on a Mac. […]
Our approach with Chameleon was to use native AppKit constructs in
the context of UIKit. The glue that holds these two frameworks
together is Core Animation.
It ends up the current Mac version of Twitterrific was built using Chameleon. Great stuff, and they’re raising money to fund further development.