Thanks for your fast replies!
It's and LCD (A fantastic Xerox 19 inch that I have used wo any fault with perfect picture quality for 8-9 years!), and I have read the wikipedias about the "burn-ins" and "imagage percistanses" you so helpfully gave me. So I am hoping it will go away. But even for LCD's it seems it could be permanent. *shudder*
It's gone about 3 hours now since the "horror" was discovered, with monitor turned of 1 hour of those 3. No DD of course yet.
The trace is left, I can't say for sure if it has diminished. I will try to make a "white screensaver" to help the problem go away, and have the monitor turned off as much as possible. Then, hopefully me and my DD addicted wife can go about with our normal lives again. That is; abandoning our mature responsibilities for DD.