Yes, I've heard that British paratroopers took some prisoners after one battle and found three of them to speak perfect English with an American accent, and a sergeant then gunned them down because he saw them as 'traitors' or something like that. Here's an article about it from the 90s:-
http://www.independent.co.uk/news/fa...s-1540755.html
I wouldn't class it as a conspiracy theory as such, because I don't think it's claimed that these 'American mercenaries' were working on order from the US govt. at the time. That wouldn't really make sense.
The US secretary of defence at the time (Weinberger) offered us the use of any pretty much military hardware we requested "up to and including the USS Eisenhower". We didn't take the US up on the offer of that help, probably for reasons of national pride. We did make use of lots of American spy satellite data that the their government passed to us though.
If there were Americans involved they were probably of Argentinian ancestry or just working for the Argentian govt. as straightforward for-profit mercenaries.