Part of me is happy. Apple pushed MacRuby before Lion and effectively killed it internally going forward after the ARC announcement. MacRuby relies on the GC capability of Objective-C which is incompatible with their new ARC baby. Just a few days ago I wanted to take a backend framework we wrote and build a command line tool to call some of it's methods in MacRuby (lots of command line parsing and Ruby has good facilities for that) but forgot we converted that framework to ARC. No dice.
This looks promising but at the same time I really wish the backend compiler was open sourced and worked on Mac and not just iOS.
It doesn't sound like they're using ObjC garbage collection, which I do not think would be available in iOS anyway:
"Our memory model, similar to Objective-C ARC in design, does not require any extra memory or processor footprint to allocate and reclaim unused objects"
Since Laurent Sansonetti is the author of MacRuby AND RubyMotion, he already said feature from RM will be backported (and hence open-sourced) to MacRuby
I'll simply point you at Laurent's email to the MacRuby-devel mailing list about a month ago. I think re-reading it in light of today's announcement could be...informative ;-)
This looks promising but at the same time I really wish the backend compiler was open sourced and worked on Mac and not just iOS.