Under the "Image" Menu
there's a submenu called "adjustments", and one of the options is "Desaturate". This will turn the layer to black and white, although the image will still be in RGB.

Alternatively, you can use "Image"/"Mode"/"Grayscale", which will convert all layers to black and white.
(Indole Ring, Fri 15 Nov 2002, 21:38, archived)

Thanks,
for all of the above ^^ :)

Oh, and EverStoned, no incidentally it wasn't.
( Mushroom, Fri 15 Nov 2002, 21:43, archived)