That happens sometimes with LCDs. It's not a permanent change like it would be on a CRT. You can blast the screen with different images in an attempt to make it go away, or you can ignore it and it will eventually go away on its own.
I've not had any burn-in on any of our machines with LCD screens (both portable & desktop) many of which have been on for quite a number of years now. Nor have I seen it on other people's system.
One thing I do notice on some screens is kind of a paler horizontal line around one or more, or all of the edges of monitors. Mainly cheaper pc lcd panels. If I look really close at my 20" Studio Display while the screen is near black with flurry saver on, I can see a very very faint thin purplish glow line around the top edge.
My personal screen settings are half brightness and screensavers are set to come on in a matter of minutes or just fade to black via energy saver.
I use an older version on my iMac G4. I like that you don't have the program show up when you Command + Tab. There's probably a setting to turn that off. Has anyone found it?