Right now I'm really enjoying Clojure- it's lockless approach to single machine concurrency is plenty, plenty fun. Unfortunately the downside of learning Clojure is that many other popular languages look downright archaic to me in this regard now. Now reading over this document, whodathunk, concurrency programming might be as fun in C as it is in Clojure!
I am missing some details though. Are these true closures? What are the scoping rules? Can you read data declared outside of your block? Can you alter it? Is the only way to keep alterations safe to use a private queue? I guess time (or a closer reading) will tell.
Is there more info available? The white paper is nice, but not too packed with detail.
Regarding the scheduler, I'm not sure enough details are known outside of Apple at this point to know what the results would be on Snow Leopard. Your milage would obviously vary when porting to some other OS. It's easy to imagine that running a PThreads app might hog some cores, or be punished inappropriately by the scheduler. These things can happen when running more than one threaded app on a system today, without GCD. This behavior, even if not documented, should be relatively easy to explore with some simple demo apps, though, without porting your entire app.
And, uhm... you will need to refactor your application to take advantage of the new API. (Was that a trick question?) Apple's guidance indicates that you can make modest changes for big gains by focusing the refactoring effort on the parts of the app that do the most computationally. For some apps this can be a small amount of work for a large gain, particularly if you can take advantage of OpenCL as well as GCD.
