If a pilot waves back, does it violate the sterile cockpit rule?
From rgl.faa.gov


  Sec. 121.542
  
  Flight crewmember duties.
  
  (a) [I omitted this, it's about noncritical (company) procedures]
  
  (b)
  No flight crewmember may engage in, nor may any pilot in command
  permit, any activity during a critical phase of flight which could
  distract any flight crewmember from the performance of his or her
  duties or which could interfere in any way with the proper conduct of
  those duties [...] such as [...] engaging in
  nonessential conversations within the cockpit [...]
  
  (c) For the purposes
  of this section, critical phases of flight includes all ground
  operations involving taxi, takeoff and landing, and all other flight
  operations conducted below 10,000 feet, except cruise flight.


One could argue that waving could distract the pilot taxiing, so according to the letter of the law, yes, you would violate the rule. 

However, I think in the spirit of the rule it's fine, since it just means that the pilot not taxiing is paying attention to the surroundings as well. Reading between the lines, it basically says


  (a) During critical phases of flight, all crew should focus on flying, not selling cooked cardboard
  
  (b) Also, don't read the paper with a croissant while asking help with a crossword during said phases.
  
  (c) Critical phases of flight are when you're likely to read the bumper sticker on another jet.


I don't think a quick wave is quite as severe a violation as e.g. reading the paper (or, as the FAA calls it, reading publications not related to the proper conduct of the flight) during take-off. Compare it with a bus driver waving to a fellow driver or one reading the paper while driving. But then, if you do park your plane in a ditch while trying to wave to your mom, don't be surprised to be held fully liable.