The WiFi versions are the same. However, in the US, there are two versions of the 3G iPad - the AT&T and the Verizon. The AT&T is the same as the European model. The Verizon model is specific to the US and will not work in Europe.
I am currently using a US 64gb WIFI & 3G iPad 2 with the AT&T gsm chip - I have not had one single problem with it working in the UK other than my navfree getting a little up set and saw a US setting somewhere but that was sorted (well I hope so lol) on registering the product. As the chaps above say I believed it was the same unit other than some Id code imprinted in it somewhere, which is what my navfree saw