A couple days ago, Daniel Jalkut wrote a quick note wondering if the future is writing native desktop apps in languages like Python or Ruby, dropping down into C/Obj-C only for performance reasons. It's been an interesting thread since then, in comments and other posts. Notably, today Bill Bumgarner responded with a long and informative ramble about how the dynamic language future is already here.
Specifically, he notes that the Python-ObjC bridge has been working for quite some time, and is even in use by a few commercial apps.
As official support for this kind of development grows, I bet we'll find more apps written with mostly Python (or Ruby) and a few ObjC/C/C++ bundles or frameworks. I really don't think this will be a performance problem - from what I can tell, performance problems in desktop apps are more often algorithm problems or I/O problems - overheads of small constant factors due to using an interpreted language are in the wash.
I've probably written as much Python (not using the bridge) as Objective-C, and my experience is that I feel like I'm solving the problem faster with Python. I particularly like the more compact syntax for common data structures like lists, dictionaries and tuples - I've written and modified a couple of very small Cocoa apps using PyObjC (including Python plugins for VoodooPad), and it feels like I'm getting away with something when I pass inline lists + dictionaries to AppKit...
In case you're wondering about what Py-Cocoa code looks like, there are a lot of great examples on the PyObjC project site. Looking at it now, it's like I'm reading Cocoa email-pseudocode, and I like that.