This is most likely not Burn In, and is still Image Retention. 20 minutes of screen wipes and some movie watching is not enough to get rid of severe IR, it can takes weeks or months to fade.
People have had a lot of success with the Pixel Flipper on the Disney WOW DVD for getting rid of IR as quickly as possible. Unfortunately you still have to be careful with Plasma TV's, 2 hours is not an issue generally but when you go 2-5 hours regularly without a really good amount of diverse content in between you can still run into issues.
I hope it goes away quickly for you. Good luck.
I've read up on that Disney WoW disc, and I'm gonna give it a try. I'll report my results when I can. Thanks everyone for the help.