Previous post is accurate. Just to fill out the options, if you purchase an iPad from Apple (or another reseller like Best Buy) you'll have a choice between a wifi and a 3G model and a choice between a Verizon (CDMA) and AT&T (GSM) model among the 3G choices. If you purchase an iPad from a cellular carrier, you'll want to go to an AT&T store. They (obviously) carry only the GSM version. No difference in prices, by the way, wherever you purchase.
If you opt for an AT&T 3G model, you'll have no problem replacing the SIM card when you return to Europe.
By the way, a wifi only model will also work in Europe where a wifi network is available or the iPad is either tethered to an appropriate phone with 3G capabilities or to a mobile hotspot that acts as a "hub" for the wifi signal via a cellular network.
I mention this only because some folks mistakenly believe they must have a 3G iPad in order to use it outside the US. That is not the case. Wifi, unlike cellular networks, is not bound by national borders.
P.S. It's likely that you were under the impression that all iPads have SIM cards because in Europe there would be no point in selling a CDMA version of the device.