I have my Logitech Harmony setup similar to Plucky. One thing you must do for this to work though is go into the Xbox 360 system blade and update the remote control setting to allow the Xbox 360 to be controlled by a Media Center PC remote.
I have my Logitech software up in front of me and the devices I have are as follows:
* TV (my 42" Plasma)
* Xbox 360
* Media Center PC (I setup a Dell XPS 420)
[When you add a device, add Computer>Media Center PC]
I have the following activities:
* Watch TV (uses Media Center PC & TV)
* Watch DVD (uses Xbox 360 & TV)
* Play Games (uses Xbox 360 & TV)
For my media center pc I adjusted the power settings to power on using the green media center button.
One thing you lose when you use a media center PC as your device instead of the xbox 360 is the 360 guide button, but you gain a WHOLE lot more (Recorded TV, Music, Pictures, Videos). To solve for this, I just used the Learn Command under the main media center activity and used my Xbox 360 remote to learn the 360 guide button. Now I have everything working great!!
My Network Project - Media Center Blog