if you have a LCD hdtv, there will be no "burn in".
if you have a plasma hdtv, there could be some issues if you watch widescreen a lot. it isn't burn in though. remember, when the screen on the plasma is black, that means the pixels are not being lit. the problem is that the phosphors in the pixels wear out over time and become more dim. so if you watch widescreen with black bars all the time, all of those pixels are not being used. so the pixels in the middle of the tv will become less bright over time and the pixels in the black section could likely be brighter, giving you a picture that is not uniform in brightness.
plasma's have gotten better over time with screen longevity, and now last an average of 100,000 hours before the screen is 50% less bright, which means there is still some life left. and at 100000 hours, you could watch 8 hours a day for many years before again its only half as bright.
i have a panasonic plasma and love it. i will usually zoom the picture in some to avoid the black bars for this reason, but really there shouldnt be any problem nowadays with plasma's. many generations ago you could likely see a difference after sometime but i doubt you would have any issues with you tv.