Your TiVo dials into UUNet and then connects to TiVo's server over the web and downloads the slice files containing the guide data (and showcases, promotions, etc). Using an ethernet adapter bypasses the dialup but is otherwise exactly the same.

People who really, really don't want to pay subscriptions, or live outside the US (Canada in particular) have scripts that scrape web listings like Zap2it and generate slice files that can be fed into their TiVos. Last time I looked at it it was terribly inconvenient.