What does this have to do with using an iPhon in the Enterpirse (corporate environment)?
You can activate it with any AT&T SIM you want. Nothing on the SIM will be altered. If you want to use it as a phone, you will need to sign up for AT&T service. AT&T does unlock out of contract phones now. Contact them and find out if the phone is eligible for unlocking, although the only supported GSM carrier in the US is AT&T, so unless you intend to take it overseas, there isn't much point.