Mike - I have a cheap Sampo 50" plasma. Very little in the way of burn-in protection and a bit prone to getting som after-image ghosting. Yet, I have had no issues with watching DVDs in their native 2.35 format with black bars above/below the image.
For a 2-3 hour movie, it just isn't going to impact your display. Newer displays from better manufacturers are even less likely to have issues.
So, I would say you are being paranoid. Just don't leave the display on one scene all night long and don't JUST watch 2.35 movies 100% of the time. Flip over to Lost in HD for an hour and some HD late night shows.
Paranoid is good of course since plasmas aren't cheap. But, in this case, it shouldn't be an issue.