It's not simply the capability of producing it, but also the materials. Don't forget that in the past, Samsung makes components, including some processors, for several of Apple's mobile products, and there's only so much material to go around. If Samsung can make more money by manufacturing some parts for Apple and using parts from other manufacturers in their devices, you'd better believe they're going to do it in many cases. And I think we all know that there's a device flying under the radar right now called the iPhone 5, along with a refreshed iPod Touch, and a rumored iPad mini for next month. To release those devices on the scale that Apple releases them requires a significant portion of the materials that are out there, along with use of manufacturing plants.
They also solved the LTE/quad-core compatibility issue with the Korean S3 that was released in July around the time the U.S. versions were released. Part of the reason for the U.S. versions not receiving that combo was not only the desire to keep the size the same in nearly all of the world (the Korean LTE S3 is slightly thicker than the rest), but also because there wasn't enough time to get the processors manufactured on that large of a scale. They could get enough made for a July release in South Korea, but not for a July release on three carriers in the U.S. (and possibly Canada and Japan) as well.
But of course, pretty much anything we have right now that pertains to any of that is pure speculation at this point.
