IMO, best way to do this is for Apple to normalize that USB port and roll out an app store so that someone like Elgato to bring their solutions to the

TV. Why? Anything engineered inside will have all kinds of issues (most notably those referenced by takeshi74). There is also a widespread belief that Apple doesn't want

TV to become a DVR as the goal is for us to consume all of our content via iTunes rentals/purchases.
Another possibility would be HDMI passthrough. Add an HDMI port and then route your existing source of television through

TV HDMI in to HDMI out to HDTV. But that's new hardware. And it's only going to be a solution for a single source of video.
An available (now) option to solve most of the switching problems is to add a Receiver to your setup. Run however many video signals you have to the Receiver video inputs and then push them to the HDTV via a single HDMI cable. That reduces the signal setting to a single button press (to put the Receiver in the right input mode). The TV would always receive video through the same (single HDMI cable) input.
Another existing solution if you don't have a receiver and don't like having to click various settings to swap video sources is to buy a universal learn remote control with macro functionality and then assign macros to your various devices. Macros will execute a number of remote commands with a single button push. I own the Universal Remote MX850 and it does this very well (it's "one remote to rule them all"). I dedicate a single button via it's LCD screen as a "setup" button and then click that button to manage the switch from one source of video or audio to another. It executes a macro that does all of the switching in a single (button) push.
These latter 2 options are "right now" solutions that don't require Apple to do anything. Thus, you could get these solutions in place today.