If you buy an iphone 3g can you just stick your ATT SIM card from your previous cell phone in it?

Hey

It depends on your location and your network For example, in the US, AT&T have a deal with Apple for a 2 year contract.

Now, you can always jailbreak your iphone and then put in the other Sim card. You will retain your number. But I can't really say if your network will trace it or not.