I also just noticed an incident of burn in on my iPad 1 when I changed the background. It can see the top bar in safari burned it and it's very noticeable. So much that I can read the words google in the search box on the burn in.
I tried re-starting to make sure it was not a glitch but on the black boot up screen you can still see it even better.
LCD burn in 99% of the time just means image retention. It takes roughly equal time to get rid of (by displaying anything else there) as it does to put it there, and it's almost never permanent.
Just turn your iPad 180 degrees and keep using it.