Update March 24, 2016: As of March 24, 2016 U.S. citizens will no longer require a tourist visa for travel in Argentina. US Embassy announcement
As of December 28, 2012 all tourists visiting Argentina from the United States ($160 USD), Canada ($75 USD) and Australia ($100 USD) must pay the reciprocity fee online before arriving to Argentina and present the receipt at immigration in the airport or border crossing upon arrival.
Here is a poorly translated online instruction PDF that the department of immigration is providing to help people navigate through this new system: How to pay the reciprocity fee to visit Argentina
Here is the main page for the department of immigration in Argentina in English
This is all fairly new so please leave your comments as to what you experienced in the comments below. I am not sure if they are punishing people that did not get the memo on having to pay online before their arrival or not…