I called work and had them check on a system we use called "timatic" it's a data base used by airlines (I don't know if TAM is a subscriber) to check entry requirements for citizens from different countries traveling to or through other contries.
US citizen transiting Brazil: No visa required providing you are leaving within 8 hours, on the next departing flight. No overnight stay is allowed, no hotel is available within the transit area. Make sure the TAM flight arrives at the same airport the UAL flight departs from. As an example if the TAM flight arrived at Jorge Newburry and the UAL flight departed from EZE your fucked. You are "strongly advised" to obtain a transit visa before going. [email]consular@brasilemb. Org[/email]
Baggage agreement:
UAL has a full agreement with TAM, meaning that not only can they accept bags from you to be transfered to UAL but if your TAM flight from BsAs to GRU fails for some reason you could in THEORY combine your TAM ticket and your UAL ticket to fly direct from BsAs if UAL had room. You can't just walk up to the UAL counter and try to fly, TAM has to send you (think of a check you get from somebody that owes you money that you endorse to a third party you owe money to)
All this being said here is my advice, check with the Brazilian consulate and see what the deal is with a transit visa, it might be cheaper and easier to obtain then a full visa. Check your bags with TAM but be prepared to have to follow up with a supervisor to get them checked through to your final destination. Go to EZE early and if you run into problems go to the UAL counter and ask them to assist you with TAM. If upon arrival in the EEUU and your bags aren't there be very nice to the UAL baggage person because I helped you a lot here.
Thank you for using the Daddy Rulz travel agency. We are also available for boliche tours during your stay in BsAs.