A few month back, Apple quietly slipped a very nice Objective-C to Javascript bridge into WebKit. Since the first commit while we were busy celebrating New Year’s Eve, it has been fairly actively developed and improved. This new API supports straightforward embedding of the JavaScriptCore interpreter into native Objective-C projects, including reading and writing variables and object members with appropriate type coercion, calling methods on JavaScript objects, and directly binding Objective-C objects into JavaScript.
It seems likely that this API is going to become public in Mac OS X 10.9 (where JavaScriptCore is already a public framework), and it might be a hint of an eventual public API on iOS. Either way, a new option for building hybrid JavaScript apps is here.