The wife and I replaced our aging 32" behemoth with a 55" Led. I really can't believe how much better games look now, we should have done this earlier - I've obviously been missing out

Anyway, is "burn in" a problem with these? I'm letting the signal determine the ratio and i'm a little concerned about the black bars burning into the screen.
At this point no. It's not a problem.
You might be able to make it happen if you intentionally just abuse the TV by pausing images and leaving them on there for days, but in any kind of normal use situation... nah. Don't worry about it.