I was given quite a fright this winter by a UA check-in agent at LHR who told me that my nefarious habit of being called by my third (and given) name would no longer be tolerated after the roll-out of Secure Flight. After reassurance on the Travel Safety forum here I decided to stick with the only forename I have ever used, and have had no problems whatsoever during several flights to, from, inside, and outside the US.