Simon Phipps, chief open source officer at Sun Microsystems, has clarified the company's business model for open-source software, including its MySQL database, suggesting that Sun's revenue hinges on delivering, "for a fee, the means to create value between deploy and scale, for those who need it."
In other words, Sun provides services, hardware, and software over and above the base level of value provided by its open-source software. Though Simon doesn't explicitly state this--he notes that "each software team at Sun interprets this model in a slightly different way"--such additional value may well include proprietary software, like the MySQL Enterprise Monitor.
This is, of course, not a break with MySQL's past, but rather a continuation of it. It's also a fair trade-off, in my view, between customers that want to seriously scale Sun technology: if Sun/MySQL can offer value-added services that augment that scale and deployment, but don't diminish others' ability to achieve these results given enough time and money, who is hurt by this?
Delivering value can take many forms, and nothing is absolutely forbidden unless it creates a barrier between download and deployment in any way.
Sun appears to be intelligently walking this line, ensuring that its software is open but not, in every case, free (as in price and as in all-inclusive-perfect-software-that-obviates-the-need-for-the-customer-to-ever-pay-anyone-for-value-received). The extra value is optional--users can still get exceptional value from MySQL, for example, without paying Sun a dime. But Sun's idea is that enough people will want this optional value that it can create a significant business with free software.