So, basically, you're stuck with retain cycles right now. My bet is that they implement a way of marking weak references rather than becoming more intelligent than ARC.
Of course, I guess I'm use to not having to worry too much about memory, either ...
Weak references aren't a panacea. It's not that uncommon to have cycles where all edges are equally strong. Any social graph ...
I use use counts to collect those cycles. ARC doesn't let you use use counts, but you don't have to use ARC everywhere. You do have to access to the underlying use counts ... MacRuby should allow access to that (it's just a standard Cocoa call) as long as it doesn't keep stray retains around that screw up my count balances ...
And I think CoreData is the answer to problems like a social graph. It does its own specialized memory management. But you're right, it's awkward to have to think about it.
"It's Ruby, you don't need to think about managing memory. Ever. RubyMotion will by itself release the objects you create when they are no longer needed. 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."