I have been having problems with my iPhone thinking I was in Houston, Texas. When posting on clients, such as Lifecast or Twinkle, or using location-aware applications, such as Maps or Exposure, I would be in the wrong place.
Yesterday I read a Google feed item about this particular issue. Apparently the 3G network is throwing off your actual location. If you disable 3G in Settings -> General -> Network, the location issue disappears.
Location is determined first by cell tower and wifi hotspot locations and then by the built-in GPS. My guess is that the assigned 3G cell tower locations are incorrect, probably programmed to a Houston location.
Hopefully Apple and AT&T will address and resolve this issue soon.