The libdispatch library is available in the FreeBSD ports collection for experimental use on FreeBSD 9.x. Several kqueue kernel enhancements are planned for FreeBSD 8.1 in order to allow the libdispatch port to be used on FreeBSD 8.x as well.
Information is available on the FreeBSD wiki about installing the libdispatch, llvm-devel, and compiler-rt ports allowing full use of the blocks extension to C with libdispatch.