The January 2010 issue of MacLife mentions the Biologic Bike Mount by Dahone. They hadn't tried it, but they thought it sounded good.
It retails for $59.99 but you can get through a reseller on Amazon for $57.75. Dahon Biologic Bike Mount for Iphone Two apps that they mentioned were the free Bike Computer from everytrail.com and the $2.99 MotionX GPS Sport. Both let you track your speed and distance, but MotionX has a topographic map and lets you email your route to friends. There's also the B.iCycle GPS cycling computer for $9.99 with more functionality. I'll be trying out all these apps and will provide my thoughts. I won't be buying that bike mount though; it's a bit too pricey.