Restoring without a sim card

I have an iPhone 3G and now have upgraded to a new 3GS. I gave the orginal phone to my son to use as an iTouch. The orginal phone now needs to be restored, but we no longer have the sim card (he forgot what he did with it after he tried using his T-Mobile card in the phone). We live in Taiwan now, so no access to ATT to get a "replacement" sim card.

How can I get this phone to work like an iTouch? After the restore, itunes will not load the phone without a sim card.