Will an AT&T sim work in an unlocked German iPhone 4S?

My boss lost his iPhone 4S in Germany. Will an unlocked iPhone 4S purchased from the Apple Store (in Germany) with an AT&T sim work both there and in the States? I'm finding conflicting info.

There are 2 alternatives to discover the iPhone 4. If your baseband is 4.10 or reduced, you can discover with a customized gevey SIM. If your baseband is 4.11.08 or 4.12.01, your only choice is a manufacturer IMEI discover. If you purchase the gevey discover, you will get an unique applenberry gevey sim with simple to use guidelines + on the internet entry to application discover mature variations of iPhones. If you purchase the Factory IMEI discover, we will formally discover your iPhone by placing it on apple's whitelist. What does that mean? It indicates your iPhone will be revealed slightly by basically offering your IMEI variety to us. We will do the relax and have it revealed by the apple company. It will remain revealed for lifestyle, there's no way to opposite this kind of discover. You can easily upgrade to more recent firmwares or recover the cell phone absolutely, but this discover will remain on your cell phone forever! It mostly requires up to 24 time to manufacturer discover an iPhone, but can take more time on vacations. We complete most opens within 1-2 times after getting your order! After its done, you only link your iPhone to the apple company itunes, delay until it up-dates and you will see a best wishes concept saying your cell phone has been revealed. The manufacturer discover only performs with AT&T iPhones! Study more about the Geveys HERE.

Well when you buy an unlocked iPhone from the Apple Store it comes with no SIM. You are responsible for getting the SIM & using it as such, but yes unlocked phones will work with any GSM carrier in the world.