Yes, AT&T prevents sideloading on the Dell Streak
I know you were just starting to feel warm and fuzzy about AT&T since the Dell Streak isn't being bogged down by carrier bloatware, but let's get back to reality.
Just as we feared, AT&T has indeed removed the setting on the Dell Streak that allows non-Market apps to be installed. On the left, the UK Streak (locked to O2); on the right, the US Streak (locked to AT&T). As you can see, the "Unknown sources" option that allows sideloading has been removed. Along with it, (un)surprisingly, is the "Running services" options that lets you control—you guessed it—running services.
You know how to get around the sideloading restriction, of course, but you shouldn't have to use a third-party solution and a computer to do something that Android phones on every other carrier can do out of the box and directly from the device.