You'll hear arguments on both sides; what I take away from the discussion is: you CAN pitch your new beer onto the whole cake, but it's BEST PRACTICE to wash it first. People have reported decent results from pitching onto a whole cake, but I don't know if there's really an argument for doing so other than laziness. It's not going to have the growth stage that creates "normal" beer flavours, you're pitching onto a bunch of dead cells and old trub, and I'd think it increases your risk of infection. The "Yeast Washing Illustrated" thread on homebrewtalk.com breaks down the process very simply - it's not hard or anything.

Good brewing!

I don't use the whole yeast cake, and I may or may not wash the yeast as the mood strikes me. I try to figure out the appropriate amount to pitch (usually a cup or two at most) and use that.

Check out the bottom of this article for a good description on why not to use the whole yeast cake:

www.mrmalty.com/pitching.php

The site also has a really good pitching rate calculator that I use all the time.

Cheers,
Corey