We bought Entel sim cards (following the advice of Siempre Viajando) and wanted to share our experience.

We initially planned to purchased sim cards at Puerto Natales, but we had enough time between our connection in SCL to do it at the airport. There are Entel and Movistar kiosks on the third floor in front of the international and domestic check-in counters. The Entel kiosk was in front of the domestic check-in counters (can't recall exactly where the Movistar kiosk was). The sales rep at the Entel kiosk spoke very good English which helped a lot as we spoke very little spanish. The Entel kiosk sells sim cards and prepaid plans, although at that time, they ran out of sim cards, so the sales rep pointed us to another store called Fotokina that sold sim cards. Fotokina is on the third floor as well, past the international check-in counters and the bathrooms.

After purchasing sim cards from Fotokina, we went back to the Entel kiosk and bought the prepaid plans. The sales rep made sure everything is set up and ready to go. We didn't need to provide a RUT to buy the sim or the prepaid plans.

Sim card: $6,000 CLP (about $8.50 USD) - accepts cash only CLP
Prepaid plan: $10 USD for 40 mins local calls, 2 GB data - accepts cash only CLP or USD

We had 3g data in Santiago, Punta Arenas, Puerto Natales, fast enough for google maps, search, email, etc.