You shouldn't have a problem using it. There are certain carriers (i.e. Nextel) who push signals to the phone and receive back programming codes that send your type of handset (model number) which then determines which feature sets and such your phone has--so it can properly transcode MMS messages and such...I don't believe Cingular does that.
Just make sure you put in the right SMSC (SMS Service Center) number in your messaging options if you decide to use it. Other than that, if the handset is unlocked, you should be golden.
Let us know how it goes ^