The DirecTV iPhone App is available now, and can let you browse 2 weeks of shows, set recordings and control more than one DVR in the house. And, it's free.
TUAW says this is much, much better than the web-based method, and works well on 3G or EDGE. It requires one of the compatible models, and can even set a recording two minutes before a show starts. If you have a DTV DVR and an iPhone, you should get this. [App Store via TUAW]