Sure, if you leave it on that one channel for 2 or 3 days straight without turning it off or changing the channel.
Or, in simple terms, if you want to burn in an image, you will have to try REALLY hard to do so with current plasma displays.
Watching movies in letterbox format, or a ball game, or the news, for a few hours will not damage the display.
You should use the plasma for a week or two and just be more careful to not leave stagnent images up on screen. But, after that time, you should be fine.
I have an older, cheaper, plasma that has no burn-in just because I'm a bit careful. It doesn't take much to have some caution and many new cable boxes and sat receivers that have VCR fucntonality, will dim the image or un-pause if the same image is left up for more than a few minutes.
Being aware is good - being concerned is fine - but actually worrying is not worth the effort. Plasma is a great deal and a great image for the money. Most of all, it is reliable and will last you years, even decades, if you just show a bit of care.