Hey, so no one could provide a definite answer and all of the stuff I found was way out of date, but here’s the proof straight from my own experience. If you have an iPhone 3GS that you somehow killed the activation on (e.g., it’s stuck on the “insert SIM” screen / Emergency Calls only) you CAN use an AT&T GoPhone SIM to activate it. I don’t know if it will work for data/voice, but at least it will get you back to the springboard!
Working as of June 23, 2010. More Info.