none. grand central is a scheme to make it easier to parallelize your code, generally so it can take advantage of two or more cpu cores. not much use on iphone devices, which aren't going to have more than one core for a long time. blocks add closure-like abilities to C. multitasking, in this context, is a way to run two or more third-party apps at once. so, basically, no overlap at all.
Apple may be laying the groundwork for specialized multicore processors on mobile handsets.
Take the just-released Macbook Pros for example. nVidia's Optimus has controls at the application-level whereas Apple's technology is at the library/framework-level. There is no user interaction to switch between different graphics chipsets; depending on need for performance/battery life, the OS does "the right thing".
