I took a look at the 3G coverage maps. It's annoying that they only let you see city-level maps rather than zooming out and seeing the bigger picture. However, they let you scroll around. According to their maps, all of Houston and its nearby suburbs is covered with 3G. It would sure be interesting to know how well it might work, but the coverage is here.

If I were to drive to Austin, for example, there are two main routes I might take. On the slightly faster I-10/Hwy 71 route (switching from the Interstate to a Texas freeway roughly halfway through the 160 mile drive), I'd loose 3G coverage once I left the greater Houston area (30-60 minutes of driving) and wouldn't get coverage again until I was all the way to the south Austin (total drive time, typically 2-2.5 hours). The Houston coverage map is a largely solid blob of coverage. The Austin coverage map, however, is much splotchier. It looks like you'd only get coverage on about half of Lake Travis, for example.

Poking around, it looks like a handful of the bigger towns, like College Station, have 3G coverage, while you're out of luck pretty much anywhere else. I'm most surprised that they don't have the Interstates and other main roads covered. Back when it was all analog with a small bit of digital coverage, the coverage maps for the state looked like a halo around the major freeways.

With zero inside information about AT&T's 3G roll-out plans, I'll bet they try to get the major interstates covered before bringing the iPhone mob into the house.