will a att go phone sim card work in t-mobile side kick?

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