DWDM certainly makes it more complicated. Of course, that same technology allows them to send much more back. (Regarding the single OC-3 mentioned previously.)
Well, DISTANCE makes it more complicated first of all. You need undersea repeaters and/or OFAs in order to get traffic from most parts of the ocean back to land, and the NSA will in many cases not want nor be able to use the "host" service providers' OFAs. This would mean they'd have to install their own, and I doubt they're going to just plop on their own regeneration site on the outside of a civilian cable.
Hum. In some parts of the ocean they must almost certainly have their own cable and then couple stolen traffic into it. I'd bet there also must exist some mini-Echelons on some Islands somewhere (like Majorca or the Azores) where they do some grooming and listening.
-TD
