The day open source became big business is the day that open-source development exploded. Yes, open source predates the moneyed interests hankering to use it to competitive advantage, but it really wasn't until IBM dropped $1 billion on Linux that companies began paying employees to write free software that the movement saw broad adoption.
That's when open source became more than an efficient way to develop software, and also became a great way to build a business.
In many ways, open source has become a critical component of the software industry because the market has largely moved from vertical businesses (i.e., companies controlling all aspects of production, distribution, etc.) to horizontal markets (i.e., companies focusing on their core competencies and depending on others for complementary functions).
Intriguingly, even Microsoft is getting into this game. Microsoft's partnership with open-source ad serving company OpenX indicates that Microsoft, too, is figuring out how to use open-source complements to loosen strangleholds competitors like Google may be hoping to throw in its way.