For any open source project with a foundation, only a small subset of developers contributing to the software wind up getting paid. Usually there are only a tiny few full time positions for lead developers like Linus Torvalds (who is paid by the Linux Foundation), or Gavin in this case.
The remaining contributors tend to fall into one of two categories
- Developers paid by a vendor in the ecosystem, who has a direct economic interest. Example: My day job is working on the Linux kernel, and Red Hat (not the Linux Foundation) pays for me to work on that open source project.
- Unpaid volunteers
And there are plenty of reasons why one remains a volunteer dev. As an unpaid volunteer developer, I am free to disappear whenever I want, ignoring all bitcoin related emails for 6 months when the day job gets uber busy, or while I backpack on the Appalachian Trail.
In open source projects, the long tail tells you that you will always have 100x more casual contributors, than steady contributors you can depend on for a timely response to a critical security bug.
Bitcoin is a very small ecosystem right now, as open source projects go. (Warning: my own predictions... I've no BF insider info at all here) As it grows, Bitcoin Foundation will probably hire another dev or two, another sysadmin, pay for some infrastructure.
Once the bitcoin ecosystem is large enough, you will start to see devs appear who are working for third parties. For example, (again, prediction, no insider info) MtGox hires their own developer, who contributes work based on MtGox's interests. Similarly, chipmakers Intel and AMD hire their own Linux kernel developers to represent their interests, and submit Intel/AMD-specific code to the open source Linux kernel.
But that is a ways off.
An open source project is a rich blend of paid devs and unpaid volunteers, everybody working together on one common piece of software, to make it better for everyone. As the saying goes, you work on an open source project to "scratch your own itch." That is the engineering equivalent of pursuing one's own economic self-interest, the essence of the free market.
Great (and clear) summary of paid developers and unpaid volunteers in open source environment. I concur. It works best when there is a methodology and requirement for transparency of compensation.