I don't know why I never thought of it and its stupid easy to do. Using either the magic lasso or quick selection tool in CS5 you can outline an object, select inverse and turn the area outside your subject into a black and white or whatever type of image.
These are just fooling around with tool images, nothing spectacular but they do illustrate the ease and potential of the tool.
For the graffiti trash can I opted to add in an extra layer, one that I used the lens blur filter in to throw everything but the trash can out of focus. Blending that layer and then cutting out the trash can in a mask again I could then turn the background black and white making the trash can stand out more. Fun stuff now to do something practical with it all. Or not.