Hi,
Usually shops are located near quite a lot of traffic, and I noticed the very same with a BV10 in a Shopping Gallery here. The televisions in the shop (located deeper within the shopping gallery) did not have this problem, but the ones on display near the entrance had it.
It seems the magnets that hold the front-cloth in place are "traffic dust magnets", i.e. collect traffic pollution over time.
This shouldn't be a problem in an home environment, unless you keep windows open all the time and live in an area of heavy traffic. Plus regular cleaning with a vacuum cleaner, set to low power, will help keep the white cloth white.