If have read before that C++ can be used to share logic. An interesting development with iOS7 is that it has a JavaScript runtime of some sort (I don't know the details) - perhaps it will be possible to use JS for cross-platform code.
You're thinking of JavaScriptCore. It allows Objective-C objects to be used inside of JavaScript (and vice versa).
It's actually a pretty cool technology. I gave a talk about it this month at Berlin CocoaHeads, and I'm gonna put some blog posts together about it soon!
