Just so you all know, you can simply put your iphone simcard into your iPad and use the unlimited 3g your already paying for on the iPad.
There is one catch, you must trim off the plastic on your sim card so that it fits in the ipads MicroSim card slot. But there is good news. They sell Mirosim to full size simcard adapters.
So basicly once you trim the simcard it will work in your iPad and then you can put it back in your iPhone along with the microsim-to-fullsize simcard adapter. I would wait until you get the adapter before trimming the simcard or you will be stuck with a microSD size sim and wont be able to use it in the phone agian until you get the adapter.
You an probably find the adapters at local phone shops or online at website like MicroSim Adapter 3FF mini-UICC Shop
once you trim the simcard and put it into your iPad it will show the ATT logo at the top of the screen with full bars and the 3G symble.
However when you try to access the internet you will still get the popup saying that you currently dont have a iPad data plan and to sign up for one.
Click Cancle and go into your settings app.
Click on Cellular Network Settings.
Click on APN settings and it will currently say "broadband" in the APN setting with blank username and password.
Change the APN to "wap.cingular"
Now go back to the settings and turn airplane mode ON.
Wait a few seconds and turn airplane mode OFF.
Your iPad is now setup to get 3G service when ever you put your iPhone simcard in it.
I decided to be the Guinea pig and give it a try and let everyone know if it works since I'm about to cancle my ATT iphone service anyways. It works so those of you with iphone plans already this can be another cost cutting option for getting 3G service when in a jam, just put the iphone sim in the iPad.
I'll post a picture of the iPad simcard and my trimmed down iPhone simcard later.