Although Steve Jobs knocked the use of a stylus at the iPad keynote, there are already products on the market compatible with capacitive touch screens. One such product that will work on the iPad is Ten One Design's Pogo Sketch aluminum stylus.
Before the iPad announcement, the Pogo Sketch was marketed as an accessory to other Apple products including the iPhone and iPod Touch. Many similar products will likely be released by third party iPad accessory manufacturers.
Despite Steve Jobs' anti-stylus attitude, even Apple itself has submitted its own patent for a touch-screen compatible stylus.