IPhone – Will I be able to use an unlocked AT&T iPhone 3GS with European carriers

The iPhone 4S is a world-phone, compatible with Verizon and Sprint's CDMA networks in the US and GSM networks worldwide. It has a micro-SIM slot for connecting to GSM networks. Most phones sold in the US are locked to the carrier you buy it from, you may have luck getting them to unlock it so you can use a different SIM. Unlocked iPhones are (tautologically) unlocked, so you can use any SIM card, but they cannot use the aforementioned CDMA networks.

tl;dr: yes.

iPhone 4 supports four GSM frequencies (850, 900, 1800, 1900 MHz). Phones like these are usually referred to as "quad-band" or "world-phones", because of these frequencies, most carriers around the world will support one or more of them.

Once unlocked, you should be able to use it pretty much anywhere that has a GSM network. You won't be able to use it on carriers that use CDMA networks as that's a different type of wireless radio entirely.

CDMA doesn't use SIM cards, so that's an easy way to tell them apart.

One other thing you should keep in mind is that iPhone 4 uses a micro-SIM. If your European provider doesn't provide micro-SIMs, you'll need to take a regular sized SIM and cut it to size.

Due to the popularity of the iPhone outside the US, even in countries where it's sold only on the gray market, it's pretty easy to find small cellphone vendors that can do this for you.