I agree with you. I want that too.
Internally we talk about two experiences: the two foot experience (in other words, your Tablet PC) and the 10 foot experience (in other words, your TV screen, or plasma screen that's on the wall or across the room).
I can see a whole bunch of scenarios where these two experiences work together. In fact, I'm about to buy a Windows Media Center to run the two TVs in my house, and I'd like to see how the Tablet PC fits into that scenario as well.
The Windows Media Center team (and others, including the Longhorn teams) are working on this. I'll try to get an interview with the teams who are working on this stuff so you can see what they are thinking of, and so you can get early access.
Being able to control what appears on the TV screen via a Tablet would be nice, I haven't tried Tablets out or the Media Center edition so I don't know if that is an existing feature.
By the way, a retail Media Center Edition would be great... I'd really like to have Media Center but I built my computer (fully capable of running Media Center) and Media Center is OEM only. ^_^;;