Okay, I just wanted to share my experience in trying to use my X-Box 360 as a set top box.
I had no trouble writing the disk, and installing it on my xbox 360. That was the easy part. Getting at&t to provision the xbox as a set top box was difficult to say the least. So follow these steps, and you should get it on the first try (this is assuming you already have the disk installed).
1) Call at&t and say "I am trying to use my xbox 360 as a set top box, and need you to provision the unit." The key word here is provision. They hear this and usually reply, "I cannot provision units, I will have to hand you off to our tier 2 technical support to do that." The lower level technical support is fairly clueless, so once you get them to forward your call to tier 2, you are golden.
2) Ask them to do the same thing. They will tell you to turn off the xbox 360 while they provision the unit. It takes a while, so they give you the option to either stay on hold, or have them call you back when they are done. After that, you will be able to use your 360 as a set top box.
A couple of notes...
-You DO need a hard drive, and xbox live gold.
-Don't give up, be persistent and they will eventually give you what you want.
-DO NOT let them forward your call to 'att connect tech'. They will charge you $50, and won't be able to help you. Thankfully, I was able to get that money refunded.
-The 360 controller kinda sucks as a remote. Buy an xbox 360 universal remote online.
Good luck, and just go for it. There's nothing to lose.