There have been rumors of the NSA and others using those kinds of MITM attacks, but to have it confirmed that they're doing them against the likes of Google, Yahoo and Microsoft is a big deal -- and something I would imagine does not make any of those three companies particularly happy. As Ryan Gallagher notes in the Slate article linked above:
in some cases GCHQ and the NSA appear to have taken a more aggressive and controversial route—on at least one occasion bypassing the need to approach Google directly by performing a man-in-the-middle attack to impersonate Google security certificates. One document published by Fantastico, apparently taken from an NSA presentation that also contains some GCHQ slides, describes “how the attack was done” to apparently snoop on SSL traffic. The document illustrates with a diagram how one of the agencies appears to have hacked into a target’s Internet router and covertly redirected targeted Google traffic using a fake security certificate so it could intercept the information in unencrypted format.
Documents from GCHQ’s “network exploitation” unit show that it operates a program called “FLYING PIG” that was started up in response to an increasing use of SSL encryption by email providers like Yahoo, Google, and Hotmail. The FLYING PIG system appears to allow it to identify information related to use of the anonymity browser Tor (it has the option to query “Tor events”) and also allows spies to collect information about specific SSL encryption certificates.
While some may not be surprised by this, it's yet more confirmation as to how far the NSA is going and how the tech companies aren't always "willing participants" in the NSA's efforts here. Of course, the real question now is how the NSA is impersonating the security certificates to make these attacks work.