I think this has already been answered but here's my two or three cents worth. I work in a public building with many many plasma & lcd screens showing the same footage day in day out.
Permanent burn-in does exist for PLASMA and CRT displays, though you can 'bleach' the image out somewhat displaying white images and such, but burn in is a very common problem with these screens.
LCD screens however do not suffer from burn in. Peter Griffin lingering around on your mac display is whats known as Image Persistence.
http://en.wikipedia.org/wiki/Image_persistence
The effects are completely reversible - you just need to be able to cope with having your mac off for a short-ish period of time.
Working with such a variety of screens, i can definitely tell you LCD screens are curable of lingering animation stills.