Tivo makes this pretty easy with their Tivo Desktop software (downloaded free from their site).  They restrict this capability on some content (movies downloaded through Amazon Unbox, for example), but most content can be easily transferred to your PC.  You can even set those transfers up to automatically occur for particular shows.  Once on your PC, you can easily transfer them back to your Tivo (I archive some old favorites on my PC to save space on the DVR and then pull them back to Tivo when I want to watch them), or you can edit them with a variety of software packages (VideoReDo is pretty cool) or burn them to DVD.

Here's a little writeup I did, if you're curious.

I know the Tivo Desktop software works right now with the Series 2 Tivo DVR's.  I've heard that Tivo will be enabling this capability with the Series 3 DVR's in the next month or so.

Chris