dimag kharab: original iPhone was 2g, most of the phones back then had a 3G sim from cingular/ATT. Since they don't make that sim for iPhone, ATT does not support the original iPhone, which is true. However you can still use it. Have 'em activate a new simcard, or put your iPhone 4 Sim ( you need a sim adapter, as its a micro sim.). It will work.

If you want to try, ask any of your ATT friends and insert their sim, you will get service. As for activating your phone itself, if its stuck on connect to iTunes, you need to jailbreak it.