EDIT: TL;DR
18.194 works fine. 18.195 and up does not work on my XP SP3 machines. Windows 7 machines are fine.
Another update: This also affects 3D visualizations on XP - no covers when playing music using the 3d visualizations, and MC crashes hard after 10 secs of the visualization running.
Ok, here's updates.
OS: Windows XP SP3
Card: NVidia 9400 (This is a Mac Mini)
Build 18.194 worked fine. Build 18.201 and 18.205 do not.
I see no video covers in Theater view, but if I toggle the list style a few times I can get the grid view to show up, but my settings are not saved, so when I go back, the view is reset back to the "cover flow" view again.
I reverted my video drivers back to 266.58, reinstalled 18.194 and the covers returned.
I moved back to jriver 18.205 and the covers were gone. So I don't think it's the video driver. To prove it, I left 18.194 installed, upgraded the video drivers, and covers appeared in 18.104. Upgrading to 18.201 or 18.205 makes them disappear.
So I think there's a bug. Next steps?