iosiphonerestore
I read this question and was wondering if I need to have a SIM card that has previously activated an iPhone in order to activate an iPhone.
I'm inheriting my mother's old 3Gs to use as an iPod / for development purposes, and I was wondering if when I restore it to factory settings I'll need her old SIM card to activate it.
This is, unfortunately, expected behavior. Whenever you activate an iPhone (which you do as part of an OS install) it talks to Apple's servers and does various things like exchange crypto certs for APNS, etc. It also checks the IMEI of the phone to see what carrier Apple sold it to, and validates the SIM is valid for that carrier (if you buy an unlocked iPhone in a country where you can buy either locked or unlocked phones the vendor will actually copy down the IMEI number of a normal locked phone, and then enter in their system where it gets pushed to Apple's servers as an unlocked phone, there is no physical or configuration difference on the phones).
The easiest way to get it activated is (as you surmised) to use an AT&T SIM. You mentioned you have an iPhone 4, you can activate it with that SIM. There are plenty of companies that sell microSIM to SIM adapters, but if you want to do it right now you can just place the micro SIM directly in the iPhones 3GS's SIM try with tape. I would not recommend doing this on a device you intend to keep the SIM in for a an extended period because the SIM is likely to come loose (and jam the SIM tray), but if you are just inserting it, activating it, and removing it you should be fine so long as you are pretty careful with how you apply the tape and not shaking the phone too much.
The iPhone can't actually read messages from the SIM card, you need to back it up to your machine and set the iPhone 4 up with the 3GS backup
Best Answer
No - any valid SIM from the correct carrier will activate a phone that is locked to one carrier.
In the rarer case - iPhone 3G was sold or is now authorized unlocked in which case, you don't need any SIM to activate the device after a restore operation.
If the device is carrier locked (in the US - AT&T is the likely carrier), you can either buy a valid SIM from the correct carrier. A prepaid line or locating the SIM that was last used to activate the phone are good options for low expense.
Apple has articles that can guide you in this process:
Using an iPhone without a wireless service plan - This covers setting up an old iPhone like an iPod touch.
iOS: How to back up your data and set up as a new device - The end of this covers restore, set up as new and more explanation of carrier plans.
About swapping the SIM card
iPhone: Troubleshooting authorized unlock issues