I am still catching up on my RSS feeds and spotted this on Engadget HD: Embedded Automation have launched a new addin for mControl that enabled two way control of Media Center. With the addon you can control Vista Media Center and extender sessions from mControl, so you could write a macro that pauses Media Center playback if the doorbell rings or any other scenario that works for you.

More info on Embedded Automation's forum

mControl adds two-way control of Vista Media Center and Extenders - Engadget HD