This is a perrenial question that (I think) is very important to a lot of people.

I wish there were some kind of website or book that would answer the question, "Who is getting paid to work on open source, and by whom?"

There is good data for the Linux kernel, for example. On lwn.net you can always see the % contribution of each company. Presumably you could mine the kernel git repo to find out how many people that actually constitutes.

Getting this kind of data for other large/famous projects should be possible.

If anyone has pointers to something like this, please speak up.

EDIT: I guess an important metric would be, "For Company X, what % of code is open source vs. closed source." It would be nice to work at a company that is 100% open or reasonably close, other things being equal (which they aren't, but my point still stands).

EDIT2: I think providing a centralized site that collects these metrics and also allows people to submit them for smaller project/companies, would be awesome and very useful. Possibly even Kickstarter worthy.