One of the 'features' that kdm has (although it could just be by accident) is that it doesn't clear the screen after logging in. What this means is that if your *dm background is the same as your desktop background, it does not flash and stays constant while the desktop is brought up - this is a nice effect.
Well, I think part of the problem is the themed greeter paints it's background on a full screen window instead of the root window. Since the theme format doesn't have the concept of "setting a background", but instead has the primitive "show a full screen image, and then show other things on top of it", it's not clear what the right way to go is. We could 1) If an image is full screen set it as the root window background, too 2) add a new "background" primitive to the greeter and always set it as the root window background 3) make the canvas window track the root window by default and set the background outside of the theme before starting the greeter 4) switch to using the plain greeter by default (and making it more themable) 5) write a new themed greeter 1 is the easiest short term hack, but I think 4 is probably the right way to go long term. We could get a lot of themability, I think, by shipping a custom gtkrc file and glade file. Anyway, we're not going to tackle this for FC7, so I'm going to close this out to get my bug count down to a manageable level.