Yes it is, in fact
But the way it works in the xbox is the same way it works in a regular ATX PC.
So, it has an internal relay that "bridges" the connection between the Red and Blue wires.
(this is known as "momentary contact-closure")
All you need to do is use "vampire taps" to tap in to the connector that goes to your ATX power-on header on your HTPC Motherboard and it will work fine.
I'm using two of them with my home-built XBMC PCs.
The "Pro" version of the Xir is better set up for use with a PC, where as the "easy" version is plug and play for an Xbox.
Also, the "Beta" version 2 allows "learning", so it will work with any remote.
I use an xbox DVD remote kit with both of my Windows XP XBMC machines, following the instructions I wrote here:
http://forum.xbmc.org/showthread.php?p=352393
So my Yamaha receiver remote, programmed to control an RCA DVD player, can turn my XBMC HTPC on and off.
-Wes