I use a skin on my keyboard (sorry, can't remember the brand) and I find it significantly reduces touch typing ability. The skin is quite thick, and rides slightly above the keys, so you have to hit the keys hard to register. I confess I prefer no skin, but OTOH the keys remain pristine, and I don't run the chance of frying it with my drink. If you can take a look at skins, pick the thinnest, most flexible one you can.
EDIT: I have also debated hitting it with a hairdryer on LOW when installed, so that it forms better to the keyboard. Haven't had the nerve, though.
[ 01-23-2002: Message edited by: The Ginger Rat ]