I wanted a simple gallery header, so I used the technique from my primer for this one. Once I had duplicated the original colour image, I desaturated the copy and adjusted levels, made a quick 'black' (ie masking) squiggle in Photoshop CS quick mask mode (with the help of my Wacom tablet from my recent birthday :o)), back to select mode, invert selection, small feather, delete and flatten.

A variation of this approach is to simply select and copy the coloured bit to a new layer, make the base layer B&W, and flatten to produce the selective colour product.