Here's some delightful news to start off your work week. When unlocked, T-Mobile's Dell Streak 7 works on AT&T's 3G and HSPA+/4G network!
This sounds like an amazing feat considering that both GSM networks use different frequency bands for their 3G/4G networks (this is why you only get EDGE when you unlock your AT&T-locked Dell Streak and use it on T-Mobile), but it turns out that the Streak 7 actually supports HSPA+ on both 1700/2100MHz (AWS) and 850/1900MHz. That's why there's no hackery involved here—no rooting, no modifying the build.prop, no unearthing hidden settings. Just insert an AT&T SIM card, enter the unlock code (more on this below), and relish the 3G/4G icons that appear in your Streak 7's status bar once the signal is acquired. Go ahead and click on the photo above for a better view.
Here it is on AT&T 3G:
There's no way to disable 4G on the Streak 7 (or any other T-Mobile device) because 3G/4G use the same frequencies, so the connection switches between the two depending on the network coverage at the time. If 4G isn't available, it'll automatically fall back to 3G.
Now on to the unlocking bit.
The Dell Streak Unlocker software that I wrote about a few weeks ago as one of the ways to unlock the Streak doesn't work with the Streak 7, but UnlockStreak (the company who makes the software) will send you an unlock code by email.
The Streak 7 unlocking service costs $49 and requires the IMEI, a unique number that can be found on the device's flip-down side door (the one that covers the SD card slot, SIM card slot, and reset pinhole) and retail box. The unlock code is based on the IMEI and generated with a special calculator. It'll take 1 to 3 business days for UnlockStreak to send you the unlock code, but once you have it, you insert another carrier's SIM card, enter the code when automatically prompted for it (as shown above), and tap the "Unlock" button. Within seconds, you'll briefly see confirmation that the unlock was successful and then you'll being taken to your home screen.
The company offers a money-back guarantee and assures customers that unlocking the Streak 7 does not void its warranty, so it's really a no-lose situation. The fact that the tablet is compatible with AT&T's 3G/4G network makes the service even more valuable, as you don't have to be a T-Mobile customer to get 4G on your 4G tablet.