I have heard that some have raised them without letting them over eat like they tend to do and have had them lay eggs. But their not meant to reach laying age!!
Cornish X (the usual meatbird) are hybrids, meaning their parents are not cornish crosses. They use a cornish and a white rock, I think, although my memory is very fallible on this point. Even if you manage to raise the cornish x's to laying age, which some people report they have, they will not lay eggs that will hatch to be cornish x's. Also, I'm not sure whether the roosters would be able to fertilize the eggs due to their weight issues (like some commercial breeds of turkeys that have to be artificially inseminated because they are too large to mount a hen).
some breeds of meat birds lay eegs. just not very many and they rarely go broody from the research i've done. the most common meat breed the cornish x is a cross of a few meat breeds that grow fast and large. they actually out grow their ability to breed before they reach breeding age. they just get too big. shortly after they get that old they start having health problems due to their size and usually die from those problems. this information is from what i have read on various sites, so if i'm wrong about this others please correct me.
bustermommy has mostly got it. Cornish cross are hybrids, and about fifty years ago some of the original ancestors were cornish and rock, but the current breeding stock do not resemble those breeds whatsoever. They have selectively bred the birds for fifty years and do not disperse the breeding stock, so unfortunately the cornish cross as you get from the hatchery at the moment cannot be reproduced. You can however raise them to a sexually mature age if you limit their feed and have some luck. The hens do not lay well. There is a thread somewhere on here describing how someone raised one of the hens to laying age. She laid around ten eggs and then quit, but he did manage to hatch some of the chicks. I'm not sure how they turned out, they were only a few days old when I read the thread.