Apple has helpfully created a knowledge base article describing the process for creating an App Store account sans credit card.
The process involves making sure you're not already logged in with another account, then choosing App Store from the menu, downloading a free app, choosing "none" for the payment type, and responding to the confirmation menu.
While this might also work for people stuck in countries where antiquated licensing agreements prevent apps from being available (like Skype in Canada! Ahem!) Apple probably intends it for users in the US without a credit card, not for cross-border hopper-shoppers. Right?