From the 300+ New Features list for OS X Leopard:

> DTrace
> Monitor virtually any aspect of your application with DTrace,  
> integrated into the Darwin kernel. Java, Ruby, Python, and Perl  
> have also been extended to support DTrace, providing unprecedented  
> access for monitoring the performance characteristics of those  
> languages.
>
> Scripting Bridge
> Use Objective-C, Ruby, and Python programs to automate Mac  
> applications. The new Scripting Bridge enables them to easily  
> generate AppleEvents using a concise, AppleScript-like syntax.
>
> Cocoa Bridges
> Use Ruby and Python as first-class languages for building Cocoa  
> applications, thanks to Objective-C bridges as well as full Xcode  
> and Interface Builder support.

and as previously announced:

> Ruby on Rails
> Work in a developer's dreamland. Leopard is the perfect platform  
> for Ruby on Rails development, with Rails, Mongrel, and Capistrano  
> built in.