All the US carriers, as well as most South American carriers, I have found to be very diligent in checking this.

On the contrary, I continue to be surprised just how casual BA is (as I've mentioned before - but I figured they'd eventually catch up).

Flying from NCE via LHR a couple weeks ago, unlike other carriers I had no trouble getting a boarding card online. About the only place I remember even showing my receipt (which is in my old passport), was at check-in at NCE. And they didn't explicitly ask for it... there just seemed to be a slight pause when they were finishing checking my bags and re-printing paper boarding passes, and I offered it to them saying that perhaps they needed to see it before the computer would let them continue.

And that was it. I'm pretty sure no one ever explicitly asked me for it in the whole trip, and other than at initial check-in I didn't even present it.

As on previous flights, there was an announcement about an hour before landing advising that if there are any US, Canadian or Australian passengers who do not already have their "visa" (yes, they called it that even though it isn't) for entry to Argentina, to please let cabin crew know so that ground staff can meet them to assist.

As it happened, one of my bags wasn't loaded in LHR, so the CSD advised me to look for ground staff the moment I got off the plane, which I did... She confirmed that one of my bags was still in LHR and that a colleague would organise with me later. She then noticed I had a US passport in my hand, and asked if I had my reciprocity receipt with me, which I assured her I did - and she said that I'd be surprised how many people arrive not knowing about it. Which, at that this point, I am surprised there are any.

I think BA must have a way for their ground staff to sort this out without the airline getting fined, otherwise I can't believe they'd be so casual about it all. I'm still a bit surprised, though.