First off, there's nothing to be scared of. Many people have modified their TiVos and have been living happily ever after. I don't want to get too off-topic, just a couple of points:

• Yes, TiVo is Linux-based. Complying with the GPL, TiVo has published their kernel changes, as mentioned in my posting.
• I'm talking about Series 1 Tivos only. The series 2 TiVos you can buy nowadays can't be modded as easily.
• If you get a network card from 9thtee.com you can program your TiVo from anywhere on the web, extract movies to watch on your laptop and relay IM messages to your TV, just to name a few. There's no way back once you get used to that.
• TiVo is the only always-on box in my appartment -- I'd like to get perl to run on it to run daily tasks talking to servers on the Internet. I'm using Tcl right now, but it's a pain.

I'm gonna start tinkering ...