All though LCD's don't have a burn-in problem they can get was is called image persistence.  I discovered this when my LCD TV, which I use as a second or cloned monitor had what looked like burn-in.  Needless to say it about brought me to tears. 

I went on the web and found that it's normal and can be corrected.  Was I ever happy.  The link is just one of many that I found.  Don't remember ever seeing a post before on this so I thought it would be nice just in case you ever notice it happen to you.

http://compreviews.about.com/od/monitors/a/LCDBurnIn.htm