If you've got a "Series 2" DirecTivo (DirectDVR with Tivo service), you just might want to check out these two links. Hacks are out there to enable the USB ports, fire up Ethernet connectivity and many more goodies:
Feint of Heart: PTVNet – TivoWeb, bigger hard drive, on-screen caller id, the works
I'm seriously considering going with an upgrade and throwing that ">PTVNet thing in there.
Hat Tip: PVRBlog