A while ago, Alain Fischer announced his new ObjectiveC plugin, 
allowing Squeak to use Apple's Cocoa and other ObjectiveC libraries.  
Todd Blanchard and I have since done some further work on it, and it's 
at the point now where it can begin be used to build Cocoa UIs from 
within Squeak.  As a quick test, I built a native OS X UI for the 
system browser, which you can see in this screenshot:

http://beta4.com/~avi/browser.jpg

The code is on SqueakSource:

http://kilana.unibe.ch:8888/ObjectiveCBridge/ObjectiveC-avi.70.mcz

You can get a prebuilt plugin (for use with Ian's 3.7 VM) here:

http://beta4.com/~avi/ObjectiveCPlugin

The browser demo can be run with "CCBrowser test".  It requires this 
nib file:

http://beta4.com/~avi/CCBrowser.tgz

You need to untar that and place it inside 
Contents/Resources/English.lproj of your VM application bundle.

I'm announcing this partly because I've run out of steam on it for now, 
and am hoping someone else will take it the next step of building UIs 
for the various Squeak tools (browsers, debuggers, workspaces, 
inspectors, etc) in Cocoa.  A custom NSMorphicView would also be cool, 
although might be pretty tricky.  Anyway, if someone does try to take 
this on, I'll be more than happy to answer any questions they have 
about the underlying bridge code.

Cheers,
Avi