I just spent the last couple of hours discovering that my U-Verse gateway’s DNS server (serving the internal network) decided, for fun, to persist old IP addresses for some of my computers. The end result of this was that, while their external/NATted access was fine, local network services that were reliant on DNS would fail (since those machines had been allocated new IP addresses). So, for example, Windows file sharing would still work, but pinging or trying to use P4 would fail.
I did find the awesomely named post, “The ATT U-verse 2Wire 3800 HGV-B. I am not a fan…” detailing other problems with it. Needless to say, I am not a fan of it either. I got this resolved (by simply locking those machines to the “incorrect” IP reported by DNS — my will to live was figuratively destroyed by this point), but the next time I run into a problem with it, I’m just going to shove another TomatoUSB-powered router behind it (to replace it as a wireless access point, basically), stick it in DMZPlus mode (more info here), and be done with it.