Yes, like every company that sells anything with a warranty, they also plan production around ensuring the have replacement units for service replacements (in the USA it is effectively a legal requirement of the Magnuson-Moss warranty act that a company plan stock of parts or replacement units around their warranty term limits). They also send the malfunctuoning or damaged units they receive back to the factory for full factory refurbishment and those go back into the service unit stock.
A neighbor of mine recently went in to one of our local Apple stores with an iPhone 3Gs and was able to get an out of warranty exchange for it.