I have come across a book that claims that alterations and augmentations to GPL works can be kept close-source as long as these are not redistributed into the wild. Therefore, customizations of websites deriving from GPL packages need not be released under the GPL and developers can earn profit on them by offering their services to their clients while keeping their GPL-based code closed source at the same time.
(cf. Chapter 17 of WordPress Plugin Development by Wrox Press).
I've never realized this, but essentially, by putting restrictions on redistributable code the GPL says nothing about what can and cannot be done with code which is kept private in terms of the licensing model.
Have I understood this correctly?