OK, I've done some more testing:
It's definitely nothing Media Center is "busy" doing - there are no thumbnails being generated, files being imported, downloads in progress, or tracks being analyzed.
When it's using a high amount of CPU and sitting "idle", as soon as I minimize MC it drops to 0% CPU usage, and returns when it's visible again.
I cleared out all the files in my zones, and minimized the file lists.
I then set up seven new library views - completely unmodified versions of all the audio presets. (excluding the 3D one)
With only my five empty zones in the left pane, and these seven library views in the right pane, the tree minimized, and the two panes with a 50/50 split, after cycling through them Media Center still idles at 11-17% CPU usage in most of these views. (it jumps from ~11% to ~17% every time Task Manager updates)
It seems like there might be slightly higher CPU usage when there are more covers being displayed in a view, e.g. the Genres view where there are lots of stacked covers - it hit about 19% CPU there.
If I close Media Center with all of these view open and re-open it without touching anything in the UI at all, it settles at roughly 1-3% CPU usage.
If the only thing I do is click on the zone headers in the left pane once each, returning to the first zone, CPU usage jumps to 6-8%.
If I go through the audio views, it's back up to 11-17% with very poor scrolling performance.
I know that's not really helpful, but I can't think what could be causing this.