They will not need an AT&T sim card redsn0w will activate it for them. There is an option as you progress through the jailbreak that will allow you to check a box that says activate. Below is from Big Boss's site.
3.1 Jailbreak for 2g/3g
Author: BigBoss // Category: Jailbreak
Pwnage v3.1 for iPhone 2g / 3g and first generation ipod touch for osx is out. I updated my pwnage guide for this release. 3gs/ipod 2g still require RedSnow which is not out yet. Before you rush out and upgrade, I still recommend staying on any 3.0 firmware you are on for as long as possible. The 3.1 firmware is not worth the hassles of upgrading. But if you have one of the two devices above, you can upgrade if you want now.
From dev team blog:
1. GOLDEN RULE: If you are using a 3G iPhone with ultrasn0w and rely on ultrasn0w to obtain cellular service, then you should only upgrade to 3.1 with a PwnageTool created .ipsw. - Stay away from Apple’s direct updates as described here and here please get up to speed on the whole subject by reading the information contained in these posts.
2. If you have an original iPhone (1st generation) then 3.1 unlock works with this PwnageTool release. iPhone 3G users upgrading to 3.1 will need to continue using ultrasn0w with a PwnageTool created 3.1 .ipsw
3. Please read all parts of this post before downloading and using these tools.
4. Read items 1, 2 and 3 again and again.
5. At the bottom of this post are the bittorrent files for the 3.1 capable version of PwnageTool.
6. This app is suitable for the recent 3.1 release.
7. This version of PwnageTool will NOT work for the iPhone 3GS.
8. PwnageTool WILL work for Original iPhone (1st Generation), Original iPod touch (1st Generation) and the iPhone 3G.