You may have thought GNU/Linux was written by idealistic Unix Gurus camped up with a bunch of Jolt-Colas in their mom's basement, but a recent report from the Linux Foundation states the opposite. Since Linux kernel version 2.6.11 in Mars 2005 the number of developers has grown from 483 to 1,057 in version 2.6.24 (January 2008). However, the number of sponsoring companies has also grown from 71 to 186 in the same time.
The major contributors aren't Mom's Basement Inc. either. Companies like Novell, IBM, Intel, SGI, Oracle, Google and HP rank among the 20 largest contributors (counted in number of sponsored changes, and here sponsoring means paying employees to program those changes).
This is just the Linux kernel (some 8.5 - 9 million lines of code). However, the Linux kernel in itself is of little use to anyone. You have to add the GNU part of GNU/Linux, consisting of commands like fdisk, aspell, bison, ghostview, and wget to that, and you'll be looking at a much larger number of lines of code. If we go even further adding programs from other projects (like the Mozilla project's FireFox web browser, or the OpenOffice suite) more lines of code are added (for exact numbers see ohloh.net), and we're still talking about programs supported by large companies (IBM, Sun, etc).
To sum it all up: no, GNU/Linux is not being written by enthusiasts in the basement anymore. It's being written by large corporations for competitive reasons. Hardware manufacturers wants to make sure Linux will work on their hardware, software companies can be anything from Linux distribution owners (Red Hat, Novell, MontaVista), use embedded versions of Linux in their consumer hardware (Sony, Nokia, Samsung), or for other reasons (for instance Volkswagen uses Linux for in-car networking between different components).