Either we take the potential for transparent MitM attacks seriously, or
we do not. I'm sure the NSA would prefer "not." :-)
I doubt the NSA is interested in MITM attacks which can be spotted by
comparing key material. 8-)
Doubting that the NSA might be interested in any given technique is, of
course, good for the NSA. Our national security people have been known
to use imperfect interception technologies when it suits the task at hand.
Do people here really so quickly forget things? There was a talk on
Carnivore given in 2000 at NANOG 20, IIRC, and I believe that one of the
instigating causes of that talk was problems that Earthlink had experienced
when the FBI had deployed Carnivore there.