fsaravi,
I took my Powerbook to MacStation in Buenos Aires (
http://www.macstation.com.ar/). They accepted my AppleCare plan and did a diagnostic on the machine. At first they told me it was just a software problem so they re-installed the operating system. After I played them a recorded sound that my laptop was making, they took it for another day and replaced the hard drive, covered by the AppleCare plan. So, you can rest assured that there are several Apple service centers in Argentina, though I think they are all in Buenos Aires (see the link another user left above, it has many Apple authroized service centers). The only bad thing, as with everything, is you have to argue a bit to get them to cover any repairs, but in the end they do cover them and repair the machines.
As for registering, I would recommend speaking with an Apple representative as I am sure you need to register your AppleCare plan, but not sure how to if you bought it here (I bought my machine in the U.S. and registered it there).
Hope this helps.