Neutral Density filters are sometimes used sometimes used to prevent over exposure by reducing the amount of light that enters the lens. Useful outdoors in bright sunlight. Often, it would be seen on big DSLRs to achieve motion blur on slow shutter speeds/longer exposure times. I believe the key Nokia phones have had ND filters since the N73? You’d have to keep the camera still of course. This way, all the static parts of the scene stay still but all the moving parts (usually water) can produce an creative dreamy like effect.
In some of the new pictures from the Nokia 808 gallery, posted as part of the 808 post yesterday, you might have seen several examples where Nokia manually forced the ND filter on to use the long exposure time to blur the water. Not sure how much control they had over exposure times.
Check the samples out here: http://www.flickr.com/photos/nokiaofficial
I’ve scheduled this post to appear right about now. I’m actually in the middle of an exam but I won’t be here for most of the day possibly.