Activating old iphone?

You can reactivate your first generation iPhone with the SIM card from your iPhone 3G, but not by simply inserting the SIM card in your first generation iPhone followed by powering it on. The SIM card must be "married" to the iPhone's IMEI number, and the only way to do this is via the iTunes activation process, which you can do with iTunes on your computer.

I had the same experience. I had to return to the AT&T store and get them to activate a new sim card. They said that a 3G sim card will not work in a first generation 2G iPhone. It took less than 5 minutes and I was up and running.

They said that a 3G sim card will not work in a first generation 2G iPhone.

If I stop by the AT&T corporate store nearest where I live and request a new SIM card for my iPhone 3G without indicating which phone the new or replacement SIM card is for, they will not ask me for which phone, nor reach into a drawer to pull out a specific SIM card for the iPhone 3G that is different from the SIM card for other phones.

After my first generation iPhone was de-activated when my iPhone 3G was activated at the AT&T corporate store where it was purchsed, I re-activated my first generation iPhone with the new SIM card included with my iPhone 3G and used my first generation iPhone with no problems, followed by re-activating my iPhone 3G with the same SIM card with no problems.

There are a number of folks who had their new iPhone 3G activated with the SIM card card at an AT&T store which they had been using with their first generation iPhone.