If it is a standalone Tivo or Tivo series two, no problem, the OS has built in drivers for certain USB to ethernet adapters (3.2 or 4.0 OS). There are websites that list compatability...

Linksys USB100TX, Belkin USB F5D5050, SMC 2208USC/ETH, among many others are reported to work... The Linksys USB200M does not.

If it is a DirecTivo (3.1 OS), like you are talking about, then the OS does not support these USB network adapters. However, there are well documented methods of writing the known adapter drivers onto the DirecTivo's OS. Some of the guide data is downloaded via satellite, but some people also have reported loosing season pass and extended scheduling features, while others reported their unit downloaded just fine if you ditch the phone line without activating the USB ports. If you rely on only satellite for guide updates, you will loose future OS updates and the ability to order pay per view. The hack in particular that you want is the Monte method for 3.1.1b

There are other issues such as backdoors for skipping the phone based setup, escape dialing sequences to access the ethernet based Tivo service, etc...

start looking at
www.pvrblog.com
www.tivocommunity.com
www.tivostuff.com
for a group of people who have the time and experience to talk about these things if you are still interested.