From the CRT days you've probably heard about screen burn-in, but there's sort of an LCD equivalent, what's known as image persistance. Nothing's technically burned in, but the pixels on the display can appear to be ghosting an image that's been displayed an inordinate amount of time.
I bring this up because this has been happening with my Powerbook display. Tiger's great and all (efficient, gets out of the way, Classic support, etc., etc.), but its one problem is its blindingly bright menubar. It's leaving a ghost on my display when I switch to fullscreen apps, and so does TenFourFox's address bar and back button, which are displayed probably more often than is healthy (Oh, look! Another cat gif!). I haven't seen this on any of my other displays, so maybe my Powerbook's is uniquely bad, but if this is a problem for you here are a couple of things that fixed it for me.
First I tried what Apple recommends, which is to switch your screensaver to an all white background and run it with your screen brightness turned almost all the way down overnight. After a couple of nights, I maybe noticed a little change but it wasn't satisfactory, and then I remembered a munubar utility called MenuShade and installed that.
MenuShade creates a shaded band across the top of your screen where the menubar is, giving the illusion that the menubar brightness is turned down. This obviously is a problem when switching to a fullscreen app--the shaded band is still there. However you can exclude these apps in MenuShade's preferences, though for some reason it doesn't work with VLC (UPDATE: you have to label it "VLC media player", i.e. the process name as it appears in Activity Monitor). Shrug, I use Mplayer anyway.
So I've been running with MenuShade dimming the menubar, and I've also started using TenFourFox in fullscreen mode to change up the placement of the address bar, and after a few days of normal use I saw a big difference, and now after about five days the ghosting is almost completely gone.
There's another utility to deal with Tiger's menubar called MagicMenu, which autohides the menubar, but it causes a lot of bugginess and misbehavior in certain applications. Not recommended unless you like slamming fist to keyboard.
Do any of your 'Books or iMacs have this ghosting problem?