So I have found a simple way to mod your At&t Sim card to fit into the iPad 3G and work on the 3G network without activating a new data plan.
Simply use an exacto knife to trace out the mini-sim on your iPhone SIM...
Once the SIM is in the iPad you need to enter Settings->Cellular Data->APN Settings
Change the APN to "phone" without quotations.
Another way is to enter Carrier testing mode in iTunes and enter a custom ipcc file. I personally used Stealth's ATT_US_STEALTH.ipcc, worked like a charm!
It has been confirmed that this mod Is working for Rogers in Canada and also T-Mobile here in the US which is great news.