Can you use your att sim card in any att phone?

That depends. Modern "World Phones", hybrid phones like the iphone 4S that contain both a CDMA radio and GSM radio are technically possible, though Verizon won't activate any iphone not purchased from them, since Verizon iphones are "white listed" to the network and sim locked to prevent using them on domestic GSM carriers. On the other hand, you can take a "World Phone" such as a Blackberry from Verizon and use it with an AT&T SIM card, as long as it is SIM unlocked, chances are you will only use the GPRS network since most Blackberrys on Verizon were limited to that. Newer Verizon Blackberrys like the 9830 have modern GSM radiOS to handle more.