Commenting on the differences between the business models of Red Hat and Acquia, Matt Asay really nails a point: there's value to be added by a company simply packaging and redistributing open-source work, but it's greatest where there's intimidating complexity to be managed. Both Red Hat and Acquia provide reliable, solid packaging of underlying open-source work (Linux and Drupal, respectively); both also provide additional value of their own, built on top of that infrastructure. But Red Hat's model relies proportionately more on the value of their packaging of the infrastructure, while Acquia weights itself more towards extensions attractive to enterprise customers.
But looking forward, he suggests that the increasing use of unpaid Linux "has the potential to disrupt Red Hat, Novell, Canonical, and other Linux vendors," and I'm not so sure of that. The trend is indeed worth watching, and the potential for disruption is real, but the value of simple, trustworthy distributions, with a legal entity answerable, remains very real for enterprises. Aside from present economic circumstances, the unpaid Linux growth is surely due at least in part to the increasingly good job of packaging and QA being done by the various distributions (not all of which are commercially backed). But mostly, I think it's the Linux-level mirror of the "adoption" benefit of more contained commercial-open-source projects. And that means much of it is use in circumstances where a commercial version wouldn't have been used anyway, and much more grows from unpaid to paid when the project at hand succeeds.
CollabNet's sponsorship of Subversion is a nice counter-example. Subversion is nowhere near as complex as Linux, or even Drupal, and Subversion is widely available in one-click installers, native libraries, and most or all of those Linux distributions just mentioned. But the CollabNet Subversion packages remain extremely popular, because they're certified by CollabNet. Many of our customers could build Subversion themselves–but that's not where they choose to spend their developer time and energy (they have their own jobs to do). Many could download some other package, or use the one provided with their platform–but CollabNet's timely updates are more important, especially for security fixes. And for enterprises, having support people, people who will answer the phone and the questions, provide the training, and provide the experience, are worth not only the trouble of using CollabNet's free downloads, but also make a support contract a worthwhile investment.