i don't see why it can't take a cingular sim card...check to see what band the sidekick is and see if it will pick up most of cingular's service before you do anything though.

most likely it wont first the danger sidekick is locked to tmobile service only then in some areas cingulars gsm is 850mz the sidekick is 900/1800 so you wouldnt even be able to get a signal. currently if you market is 850 your stuck to cingular phones or true quad band phones like the v400, v600 etc.