Some months ago at morning I figured out that top display cover on the my DSLR - Nikon D90 - was cracked. Display was working without any problems so it was just a cosmetic defect. Top display cover is a small transparent piece of plastic with glue layer on border. You can easy find it as spare part on ebay.com for a price less than $10.
Process is well descibed by the following links:
You will need hair fan, some sharp devices to remove old cover, c2h5oh for remove old glue and for good adhesion of a new cover.
Just one remark - for me it was very difficult to come off the old top cover. Finally I've used two very small needles to do it. Best results was when start from the bottom left corner. After I was able to remove old cover process takes less than 5 minutes. Now my DSLR looks like new.