Hooray! DirecTV just updated its iPad app with the ability to stream live TV. You have to be connected to the same network as your DirecTV Plus HD DVR but that's okay cause it turns your iPad into another TV.
The channels you can watch with the DirecTV app are actually pretty great. Here's the full list:
Fox Business Network
The DVRs that are compatible with streaming live TV are: DirecTV Plus DVR (models R15, R16, R22), DirecTV Plus HD DVR (HR20, HR21, HR22, HR23, HR24) or TiVo Series 2 devices. More TVs! Yes. [iTunes via DirecTV via Engadget]