No you can't use AT&T sim in the sidekick mobile until you unlock it from t-mobile network lock ...You can easily unlock your sidekick mobile using sites like MobileUnlockSolutions.com here they are rendering network unlock code for all mobile models at reliable cost .You first check that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone then freely use it with AT&T or any other GSM sim ...