First off, you'll never see it again. I highly doubt someone will turn it in. Especially if it's been turned back on and is receiving calls.
If I were faced with this situation, this is what I would do.
Call AT&T and put a hold on your account and let them know your phone was stolen. Who knows how many TXT messages they may be sending out or calls that may be charged to your account. The last thing you want to do is pay for things they are doing on it. This also stops them from making and receiving calls.
Did you happen to EVER access banking information or stored any type of personal data on the iPhone? Some people could, and would, take this information and create an idenitity. With that they could get credit cards, etc. and ruin your credit.
Also, let your contacts that you know are in your phone that the phone was stolen. It's just a courtesy to them in case they start getting phone calls from said theif.
Now most of these are the extremes but possibilities. I'm just trying to help, not freak you out.
Sorry you lost your phone but look at it this way, that 3G iPhone is freaking awesome and was cheaper than what you paid for your now stolen one.
Crawn