Yes, the open-source database market is still relatively small(roughly $200 million in 2007, according to Gartner). But when The Wall Street Journal starts paying attention (subscription required), it's clear that the opportunity is huge. The Journal doesn't get paid to be sentimental.
Regardless, as Arjen Lentz opines,
...(D)isruptive technology tends to not take over the incumbent's market, but find or develop a completely new market, and indeed take over in that space. The question then is, does the incumbent's market remain intact, or does it change/evolve naturally and perhaps shrink or even completely disappear over time. Generally, the market-dominant incumbent continues to survive in a niche (where they are obviously dominant, but no longer in the market overall). In short, the market changes and with it its rules and demands.
Leading this market transformation is Sun Microsystems. Open-source databases (PostgreSQL and, especially, MySQL) may get a significant boost from Sun's involvement:
The question now is how quickly such (open-source database) products can be enhanced to become credible contenders to take on the biggest jobs in the enterprise. Mr. Slater of LiveOps says he is interested in features from MySQL, for example, the ability to partition data--to create subsections with only specific information categories, such as months or years--which can be faster to search than large database tables storing many kinds of information.
Such improvements now fall to Sun Microsystems Inc., which earlier this year paid $1 billion to purchase MySQL AB, developer of the product. Marten Mickos, senior vice president of Sun's database group and the former chief executive of MySQL, says Sun is taking a variety of steps so users can add more data, sort it faster and get access to information more quickly.
This is also why Sun is increasingly playing the leadership role in open source. I've opined before that MySQL could become the hub of the open-source ecosystem, just as Oracle has built itself into the hub of a significant proprietary ecosystem. Assuming Sun can execute against its ambitions with MySQL (a big "if" right now, but I'm willing to give it the benefit of a doubt), Sun could corral this rising open-source database tide...
...and everything that comes with it.