Two hue/saturation layers are added one of which does the black and white conversion, whilst the other alters the picture that is being converted -- easier to show than explain :-)

For a description of doing this in Photoshop Elements see the convert colour pictures to black and white tutorial on this site (works the same in Photoshop see below)

Using two Hue/Saturation layers

Create a H/S Adjustment layer with no adjustment. Change the layer blend mode to Colour
Create another H/S Adjustment layer with Sat at -100.
Return to first H/S layer and tune the black and white effect using the Saturation and Hue sliders. Do not use lighten as it will change any end point already set .-- (method first suggested by Clay White - Thanks)

The image above used Hue -42 and sat +70

This is a very powerful and subtle technique. The variable H/S layer can have individual colours adjusted to alter the intensity of different colours.

There is a handy document with more details on this technique (and others) from Russell Brown and a movie showing the effects at:

http://www.russellbrown.com/images/tips_movies/ColortoB%26W.mov