The above image was taken on a day that was just like the one I described above. A low carpet of gray clouds, and a lack of interesting color. These are the perfect images to turn into black and white. By going out shooting on these cloudy, seemingly dull days you have to remember that your eye sees things much differently from your camera. To your eye it is a dull, uninteresting scene, but with the help of your camera (along with a healthy boost in contrast in post processing, graduated neutral density filters, and maybe a wide angle lens or two) you can turn that boring sea of gray into something interesting. Here is another example,
This image was recently published in the Capture Rochester book. This just goes as proof that great images can be captured no matter what it looks like outside!