Here is a photo of something I've been playing with.
I usually measure with a 5" x 7" notebook in my right hand, which limits me to one hand for braking and shifting and effectively gripping the handlebars. I wanted something that would let me ride and brake with both hands and give my bike more than three gears.
This device consists of a plastic translucent clipboard with the clasp relocated from the top to the side to hold the notebook. On the other side I'd place a PDA if I could find a hard case that fits mine and would open the way I want it to. It's attached to the handlebars with a device called a swing grip which I bought at my local bicycle shop.
I wanted to be able to see through it so that I could see the front wheel and a Jones Counter if I needed to use an original with the display at the wheel. I needed to be able to detach it because I transport the bike on top of my car and the wind would wreck it if I couldn't.
I'm not sure why I haven't used it more than I have; I really don't think the PDA is all that important although it would be nice to have it calculate splits.