As to the "definition" of Formation flying....you won't find one. As far as the FAA is concerned, it's contained in 91.111 where it says that no airplane shall fly close enough to another to create a 'hazard'. To me, that's a loose enough phrase for any FAA lawyer to drive a 737 through so I don't want to test the boundaries.

If ATC asks me to be a comms relay for a lost jet...hey, glad to help. If he asks me during the normal course of my route from departure to destination airport to look out the window for somebody, I'll do my darndest. The problem comes when he starts actively vectoring me or asking me to climb/descend for the sole purpose of getting me closer to another jet when I'm full of paying pax who didn't sign up for that. All the articles I've seen (including the FAA's own press release) say that there was a loss of normal separation standards. That to me says that either the controller purposely vectored/descended the 737 into a closer position for no other reason than a 'look see' or the 737 crew voluntarily maneuvered their jet towards another aircraft...either way it just doesn't pass the sniff test to me

But hey, last week I had no problem with the crews landing at DCA once they declared it an uncontrolled field and set up the approach that way. I guess that's what these boards are for, to debate different viewpoints of what we would have done from our comfy armchairs.