If you haven't already seen this thread, I would check it out. I'm currently using it as a resource for harvesting yeast from my primary.
I PERSONALLY would shy away from just racking on top of a used yeast cake with out washing it. On top of good healthy yeast presiding in the yeast cake, there are also dead yeast cells, protein cells that have fallen out of suspension, old hop particles, and potentially dozens of other "contaminants" that could potentially produce off-flavors in your second/third/fourth/fifth batch. A lot of these things are minimized when you "wash" yeast.
Washing yeast also allows you to store it for future use, in case you are not ready immediately with a second batch.
I say this humbly of course, having never tried this method, where as rsmith has clearly tried it with positive results.