Sorry for the underwhelming amount of details.  To address some of the questions asked:

- Before the switch to AT&T, 99% of the time, just entering the address without the extension would work.  The few times it didn't, I got a Ro@d Runner search listing, as T!me W@rner was my old service provider.  

- The LAN has not changed.  The RG runs to an Apple Time Capsule (802.11n), which is bridged to an Airport Extreme (802.11b/g).  Previously, the same configuration was used, but the Time Capsule plugged into the T!me W@rner modem.

- The RG is set to have the Time Capsule on DMZPlus.  The wireless feature of the RG is disabled.

- The computers in question are Macs.  Safari and Firefox are able to fetch URL's without entering the extension.  IE 8, to my knowledge, cannot.   

- The connection is good.  According to speedtest.net, I'm getting ~5000 down.  Not sure what the up is, but it is what was paid for.