Unlock Sidekick ID
I found this guide online.
Please let me know if this works!
"1) Insert the ATT SIM card into the back of the T-Mobile Sidekick ID (it should be located behind the battery).
2) Turn on the Sidekick ID, wait for the screens to load.
3) When the screen says "Waiting for Activation", hold the top left button (the "Menu" button with the three lines) on the Sidekick ID and hold the letter "L" on the QWERTY pad at the same time. After a few seconds, let go of the buttons.
4) If Step 3 is done correctly, the Sidekick should be directed to the jumpscreen and display all of the Menu's of the Sidekick ID.