My last movie starts with a section in black and white and then moves to color. The black and white was to suggest the character's mind, which saw things as if they were old movies. The one I'm working on currently is all black and white with a few key props in color: a Cadillac's red headlights, a rose in a homeless woman's hair, the Polish flag on the back wall of a bar.
Schindler's List is black and white with one key background character's coat appearing in red.
The Wizard of Oz very famously starts out in black and white and then changes to color once the action moves somewhere over the rainbow.
Raging Bull is a black and white movie with a brief color montage of 8mm-style home movies, meant to quickly depict the character settling down into a life with a wife and child.
There are lots of reasons to use this technique. As directorik says, use it by all means! Just make sure as a filmmaker that you have a reason...don't just do it to be "arty" or whatnot.