In addition to the problems of dealing with the currency situation and counterfeit money, another hassle is dealing with shady cab drivers. These people tend to be among the lowest forms of life in many of the places I've visited, but some of the ones in BA participate in a unique scam that I haven't heard of in other places. In short, you hand over a 50 or 100 peso note which suddenly becomes a 5 or 10 peso note or a counterfeit one. The bill swap is practiced and often perfected, so don't count on being able to see it happen, even if you're watching.

Actually, just prior to this post I thought I'd do a Google search on it and came up with this video link of a BA taxi driver caught in the act.
http://www.youtube.com/watch?v=0dfd9ZxxQd4

It's best to pay with smaller bills, but a good preventative measure I read about if you HAVE to pay with a large bill is to bend a corner of the bill in front of the person before paying while deliberately announcing the amount you are giving. Furthermore, you can announce the last two digits of the serial numbers on the bill if you want to be extra careful.