Jon Udell covers the Tivo HME/SDK
So many possibilities ...
On home control: I have an X10 home lighting system, and I've written a Java application that runs on a Linux server in my closet that the TiVo discovers. So I can now control all the lights in my house, and turn on the fountain in the back yard, and stuff like that.
MisterHouse via Tivo? ... hmmm.. they should probably just implement a WAP browser for Tivo. I think the remote interface would align nicely to what you can do with a phone form factor. Then you could write .NET Compact Framework etc apps for your Tivo.
(for more of my Tivo/SDK lazy web wishlist, see O'Reilly)