Great post.
I remember when @Home (before Comcast bought them) forbid using a router as well. They required you to buy another "line" if you wanted to have more than one computer use the internet connection in your house. (I also remember the good ole days when @Home allowed unlimited up and down, before they capped the upload speeds)
The benefit was that the additional line had it's own dedicated IP address (not a fake 10.x.x.x or 192.x.x.x one), so it was nice for hosting servers.
One Linksys routers got under $150, the cat was out of the bag and everyone split their internet connection like crazy.
Here are the two most relevant posts on this issue:
http://osxdaily.com/2011/05/09/how-att-detects-unofficial-tethering/
... and specifically why Android AT&T users aren't getting caught...
http://www.androidpolice.com/2011/0...unauthorized-android-tethering-and-may-never/
The problem is that there was this extremely easy hack that the MyWi/PdaNet/TetherMe developers saw in the iOS system, and they all built their apps around this. The problem was that part of the iOS system was coded by Apple to use an alternate APN network. What's really creepy is that AT&T knew that we were tethering the entire time instead of simply blocking it.
Unlike Apple, Google decided to code the Droid tethering system to use the same routing as any other app (like YouTube, Maps, email, etc). Thus AT&T is really screwed there.
I've heard mixed reports about PdaNet's stealth mode. I think we have to wait a few billing cycles to verify this. If PdaNet's stealth mode usage doesn't show on the AT&T bill for a couple months, then I'll start using it.
But I'd rather keep my unlimited plan for now.