How long ago did you get the AT&T SIM card? With GoPhone, you have to keep topping it up by a certain date/time before the minutes and number disappear. I think GoPhone has a deadline of 60 days after the minutes expire that the account becomes cancelled. From what I read on AT&T's website, any smartphone user has to have a data package.
I'm not sure if you can re-use your SIM card. It never hurts to get a newer SIM card in case something with the carrier's network changes.
The only thing I don't like about GoPhone is that it doesn't work in Canada. I found out the hard way after loading up a SIM card with some money and then tried to check up on a friend. T-Mobile works in Canada, but I'm not sure if you'd get 3G speeds with them in the US with an iPhone.
Which carrier are you with right now? I know Rogers has a data "day pass" that you can buy for $x/day. I'm sure Bell, Telus and other carriers have a similar program. If you're in a total bind and NEED data while you're in the US, it might be easier to pay your current carrier a fee and not have to fiddle back and forth with a SIM card. I did this last time I was in the US, and it worked out well.
Otherwise, if you can survive without data, you can always just use Wi-Fi for data, and then get a SIM card from either AT&T or T-Mobile for calls and texting. Just make sure you switch off any app that uses data. This is what I usually do when I travel to the US. Data is switched off on my BlackBerry, but my T-Mobile is on another unlocked device allowing for texts and calls. And, I can reply back to the US a lot cheaper with my TMo account versus my carrier's rates.