The main idea behind registration by stealth is that, simply put, registration can often become a blocker to users using a site. Registration by stealth looks to remove the formal 'registration' call to action and instead focuses on gathering the important bits of information (that the business requires) as the customer is using the parts of the site that they feel are important.
The result is a site that allows the user to move quite freely around the site and then provides them with a very quick and simple form that effectively registers them. An example is www.justgiving.com where you can sponsor without registering then right at the end of the payment process the user is presented with a cleverly designed box that ask 'Save this card for future use?', if the user then chooses to enter their password and verify their password - voila! the user is registered without any fuss.