The ATT iPad has a SIM card. All cellular iPads have the option for a SIM card. Starting with the new iPad I believe only the ATT iPads have a slot. Not completely sure.
If you mean unlocked to use on any GSM carrier, then the answer is yes. The AT&T version can use any GSM SIM card. The Verizon and Sprint ones can use any GSM SIM card as well. I believe the Verizon one can't be used on Sprint and vice versa.
If you mean unlocked to use on any GSM carrier, then the answer is yes. The AT&T version can use any GSM SIM card. The Verizon and Sprint ones can use any GSM SIM card as well. I believe the Verizon one can't be used on Sprint and vice versa.
If using in Europe, the Verizon one would be better for LTE. Anywhere in North or South America, the AT&T version would be compatible with more LTE networks.
If using in Europe, the Verizon one would be better for LTE. Anywhere in North or South America, the AT&T version would be compatible with more LTE networks.