iMac monitor leaves watermark-like window stamps after a while I'm not sure how to compile a logic title for this question, so if the title is unclear, suggest an edit. Here's a description of the problem I am experiencing:

Whenever my iMac has been on for a few hours the silhouette of the window that has been open for a while is imprinted in the screen. Moving around the window does not help, only when I turn off the monitor and let it cool down for a while the imprint is gone.

Besides a window's silhouette it also seems to imprint directory folder trees, macro, and/or icons sometimes.

I do use a screen saver, however as a developer I sometimes actively work in one window for a few hours.

Since the iMac is 'only' three years old and I haven't had this until last week, I am wondering if anybody has experienced the same issue, before taking it in a repair shop.

Kind regards
The proper jargon seems to be a burn in. This extensive post at the apple forums holds a lot of answers.