There are plenty of guides out there to upgrade any TiVo's hard drive space (I wrote here last year), but the more robust hacks like getting a web browser to control your TiVo and transferring recordings from your TiVo to your PC have historically been limited to older Series 1 devices.
On top of that, DirecTiVo owners have been largely left out in the cold when it comes to hacking, since newer devices are locked down by DirecTV. Series 2 DirecTiVos can do everything standalone series 2 TiVos can do -- DirecTV just doesn't let it happen. But last Fall someone figured out that the TiVo 4.0 OS could be loaded on a DirecTiVo with the home media options and ethernet networking enabled.
For the most part, these hacks are innocuous things that customers want, and don't take any revenue from DirecTV. Many folks (myself included) don't have a phone line anywhere near my DirecTiVo box, and would rather use a wireless network connection. Many folks (again, me too), would love to see the HMO music and photo streaming enabled on their devices. The other major feature enabled by these hacks is getting video off your TiVo, which is something many people want and should be coming soon to standalone TiVos, but again, DirecTV folks are left in the cold.
I haven't tried this hack out on my own DirecTiVo yet, but if I do, I'll certainly take photos and notes and post a full report afterwards.