I have had this problem too for a week or so, I didn't really notice when it started. But about the same time that XBMC started working it self up in idle mode, Windows Aero functions got disabled. I ran Windows troubleshooting guide wich told me that the Aero-functions didn't work because of a mirroring display-driver.After a bit of googling and browsing I remembered that Logmein (wich I have installed on all my computers) use a mirroring driver so I uninstalled it just now and rebooted and VOALA! The XBMC process is back to it's usual 0-5 in CPU-usage (10 at the most in idle).
I've had Logmein installed since my last reinstallation of Windows (a while back) so I guess it might be some update from Logmein or whatever that caused this. I don't know if this is of any help to you guys but it worked for me so now I don't have to look for a new mediacenter-software...
For the record I'm running Windows 7 SP1, XBMC 10-something. On a Intel Core 2 Duo 3Ghz, 6Gb DDR2 and a Geforce 8800GTS graphic card.
Oh and by the way, Logmein is supposed to be able to run without the mirror-driver so I'm going to try to install that now, hope XBMC continues keeping it together...