The source code for Grand Central Dispatch (libdispatch) in Mac OS X 10.6 Snow Leopard has been released: <http://libdispatch.macosforge.org/>
Prominently noted on the page is a link for a portable C compiler with blocks support (clang) and the blocks runtime (part of compiler-rt)!
The libdispatch project consists of the user space implementation of the Grand Central Dispatch API as seen in Mac OS X version 10.6 Snow Leopard. The Mac OS X kernel support for GCD may be found in the xnu project. While kernel support provides many performance optimizations on Mac OS X, it is not strictly required for portability to other platforms. However, in order to implement the full API for Grand Central Dispatch, C compiler support for blocks is required. The blocks runtime is available as part of the LLVM project.