Good news, T-Mobile customers. The AT&T Dell Streak can now be unlocked in a matter of seconds with the Rebel Sim II. I had tried many, many times to unlock the Streak with this card in the past, but what made it work this time is the firmware.
Version 1.2 of the firmware was released two weeks ago, and I can confirm that it is able to unlock the AT&T-locked Streak for use with T-Mobile (voice and EDGE). The Rebel Sim II does not void your warranty because it doesn't modify any software or permanently unlock the device; once you remove the card, the Streak is back to its AT&T-only state. Put it back in with your T-Mobile SIM, and it's unlocked for T-Mobile again. Take it out and your Streak is locked to AT&T.
Want to use your official AT&T Streak with T-Mobile (and other carriers)? Here's how to do it.
2. Punch a hole at the top of your T-Mobile SIM card. It doesn't affect the card's usability (you're just punching through useless plastic) and the hole doesn't have to be precise.
All you need to do is make sure the black square on the Rebel Sim II is exposed when the cards are stacked. That little chip is the source of the magic.
Here's a more technical explanation:
"The Rebel Simcard has an IC on the FPCB and this intercepts some of the communication between the network simcard and phone and enables the use of other network simcard's in a otherwise locked phone."
3. Slide the stacked cards into the Streak's standard SIM slot. They won't stay together very well on their own, so you can use a small piece of tape on the edge if you can't get them in correctly. There's very little room for imperfection when inserting the cards (i.e., they really need to be lined up correctly for this to work), so be patient and keep trying.
4. Once the two cards are inserted, put the Streak's battery back in, replace the back cover, and turn it on. If the cards are in correctly, you'll see T-Mobile in the status bar as soon as the Streak boots up and prepares the SD card. If you don't see it, take the cards out and try again.
And that's it.
I've got Android 2.1 on my official AT&T Streak, and it works with no problems on T-Mobile.
After I removed the Rebel Sim II and T-Mobile SIM card, I put my AT&T card back in and it was like nothing had happened.
I'm back on AT&T and fully locked to the carrier until the next time my Rebel Sim II wants to take a ride in the Dell Streak.