This is what I use, the Wallet Pen: http://www.the-gadgeteer.com/walletpen-stylus-review.html It's always in my wallet, and solved the problem of forgetting a pen. A bit pricey but worth it for the convenience. Someone needs to make it in stainless steel instead of sterling sivler. Makes great gifts.

Cool. And stainless steel would be a nice option.

-- Craig

I keep a Bullet - Fisher space pen clipped to my wallet. It is a little too fat to keep in the fold as the above one is shown, but I clip it so it rides along the top edge of the wallet, slightly increasing the profile. They are available in chrome or black and use the common Fisher refill.