It seems the wifi router comes with AT&T u-verse does not work with Apple TV. Apple TV could not find the wifi network from the build in u-verse wifi router. I found one workaround, that is to turn off the build in Wifi, use an old wifi router (D-Link DI-524) for Wifi instead. But there is still problem with the mirroring from iPad to Apple TV. Since I still have my previous LinkSys wrt54g router, I set it up as second router behind the DI-524, and connecting both the Apple TV and iPad to wrt54g. That worked. Another side benefits is wrt54g is a newer/more powerful router, and could broadcast wider range. So in a summary the architecture from internet to apple TV is something like below.
u-verse modem (wifi off) => D-Link DI-524 router => LinkSys wrt54g router => Apple TV/iPad
You may wonder why I add this extra D-Link DI-524 old router. The problem was I could not easily connect LinkSys wrt54g (the newer router) to u-verse modem. So I used the old router as a bridge. If someone has idea to make it work without this extra router, please let me know. (Email: minjie DOT xu AT gmail DOT com; or twitter @stlplace)
There is another problem with setup AT&T u-verse which is not related to Apple TV. The problem is when creating new account at their web site, they prompt for secret question/answer, it appears they don’t take any space in the answer. It was a programming error (web page text field validation) in my opinion, since I have done some web programming lately. The workaround is not to type any space. Simple enough.
This is also after using cable internet for about 8 or 9 years, I am switching to AT&T. I was using their DSL before switching to cable. The main reason is their service is cheaper, another factor is the cable had some connection issues lately.
(Update 05-07-2015) Came across this post regarding how to make direct connection from linksys wrt54gl to uverse router. I tried it but was not successful. It does not like the new local ip address 10.0.0.1 for linksys. But the theory there (conflicting local ip address between uverse and wrt54gl) seems holds, as I saw people discuss similar problem on linksys support forum.