That's very wrong... you could argue that most of the kernel maintainers are employed by companies to work on the kernel, but not all the code gets written by them, and some companies do not employ people to work full-time on the kernel, otherwise do you think the Intel driver would be in the current state?
And of course Lamson is not GCC or the kernel. Those are very special cases, look at the GNOME or KDE project... or Django for you web guys, who gets paid to work on Django? not even JKM
Hmm it doesn't really count but JBoss is open source and has paid developers. jQuery UI is maintained by a consulting group. Firefox's core developers are paid. I'm pretty sure there are paid contributors on Hadoop and Lucene.
Perhaps less well known are open-source reporting companies like Actuate (sponsors/main developers of the top-level Eclipse BIRT project), Pentaho, JasperSoft. Not sure about revenue models for others, but Actuate sells support for their open-source tools and offers proprietary add-ons for those that need to scale their deployments.