Nick Carr notes that most Linux kernel work (and I would suspect a lot of the rest as well) is now paid effort:

A new report from the Linux Foundation reveals the extent to which the most famous and successful open source software project - the development of the Linux operating system - has shifted from being a volunteer effort to being a corporate initiative. Of the many thousands of changes that have been made to the Linux kernel over the past three years, fully 73.2% came from employees working on behalf of their companies.

I like his comparison of the distribution of Kernel work to the Visa consortium.