Traditional "burn in" is due to phosphor wear in CTR televisions. Obviously, LCD's do not have phosphor, so burn in as you're thinking about it is impossible.
In fringe cases, however, LCDs have been known to have some slight image retention. Image retention, as a rule, is not permanent. If your iPad is suffering from image retention, which I doubt, keeping it off for 24-48 hours should solve the problem.
That's not quite true. LCD displays also experience 'burn in' but this should only happen if you have the same image on your screen continuously and obviously with the screen on continuously. We have large LCD monitors at work which are on 24x7x365 at full brightness and 2 have experienced burn in. The others havent suffered the same problem so I guess it all comes down to the quality of the display panel. Turning off the monitor for any period of time doesn't fix the issue.
I can't imagine how this problem could occur on an iPad as no one just has the one app constantly running and the iPad goes into standby mode automatically thus there should be no possible way for this to happen.
It would be interesting, if possible, to find out what 'make' everyone's display panel is. For example it could be that the LG panels are an issue.