Sorry Verizon Network and on the Blackberry storm uses CDMA network. CDMA network does not use any sim cards at all. So if you have a a A T&T sim card it won't be compatible with CDMA network and especially there are no slots for the sim card on the Blackberry Storm.
I guess there are still people out there that don't have a clue what they are talking about but yet still answer questions. Yes the AT&T SIM will work in the Storm but you have to get it unlocked first. For those with inquiring minds the device is a world phone and even though it is made for Verizon, which we all know is CDMA, this device also has a slot for a SIM card. This is a dual network device.
If your BLACKBERRY STORM is an unlocked Phone you can use any GSM Network Sim card in the phone.It's locked phone means you cannot use other GSM Network Sim card in the Phone.You need to unlock it.Get the unlock code for Blackberry Storm at Simpleunlocking.com and get free Unlocking instructions at Mobileunlockguide.com/blackberry-storm and Unlock it.Then use any GSM Network Sim card.