Wish I knew where I saw it, but there are groups of breeders who purposely cross them for meat. Man I wish I knew where that was!!!!! The hatch rate is low and the chicks are infertile, but they had a ton of pics...Geez where did I see that???From what I just read it seems that a cross is possible, though hatching rate particularly for hens covered by a guinea is very low.
http://www.feathersite.com/Poultry/Guineas/BRKGuinHybrid.html
I have no idea if that information is accurate. I am most certainly not a geneticist nor do I have knowledge of the evolutionary path of guineas vs chickens. It may be that they are like Horse/Donkey hybrids, closely enough related to breed but the off spring may or may not be able to breed in the F2 generation.