Your phone has to be debranded. You have to use flashtool and reflash the latest firmware or the one you want to your phone. That will debrand it, then you will root it, sorry AGAIN, to get back where you are know. Use this link for flashtool
the link should have all the questions answered for you. Use the X10a firmware since your att. Post again if you have questions.
I would recommend downloading the global generic 2.1 since att is not going to update at all. This will give you a fully working stable rom from Sony Ericsson so you won't have any issues. Just poke around the development section and you can find it pretty easy. Just follow the directions to a tee and you will be fine.
Remember that it voids your warranty with att so you will have to flash back to 1.6 att version in the event you need repairs at some point. That is also on the forum. Also make sure you backup anything you need before you flash as it will wipe everything. It wont affect your SD card though.
Sent from my Delorean using a flux capacitor!
i dont see global 2.1?
Generic version 2.1update1 build 2.1.A.0.435 (MD5 : 0C67A08A4B120EBA87E395865F431E81)
Generic version 2.1update1 build 2.0.2.A.0.24 (MD5: E592508FC714A9F2CB6BDC41914ABC81)
Generic version 2.1update1 build 2.0.A.0.504 (MD5: F3735BF4D48ED4BBA8BF5BC60CD1D34C)
these are the ones i see