Intel implemented Cilk Plus support within their own compiler but also provided open-source code for the GNU Compiler Collection. Cilk Plus depends not only upon compile-time support, but also a run-time library. Intel's Cilk Plus run-time library is dual-licensed, including under a BSD three-clause license.
Cilk Plus has been around as an Intel innovation (following their acquisition of Cilk Arts) since 2010 and since August of 2011 they have provided GCC support. Since this past summer they have readied the GCC Cilk Plus support and have been after pushing it to mainline.
While the code appears ready, Intel developers have been waiting for nearly two months for the official GCC developers to review the necessary patches. As can be seen from this latest mailing list message, they're playing a silent waiting game.
Unfortunately, this drawn out process now no longer makes Intel Cilk Plus a possibility for GCC 4.8. The GCC 4.8 development ended in November and so now Intel will be waiting until at least GCC 4.9 (or potentially GCC 5.0) in 2014 before their multi-threaded parallel programming solution will appear in this open-source compiler.