In discussions about open source software, I'm frequently reminded of the following PG quote:
To be a startup, a company has to be a product business, not a service business. By which I mean not that it has to make something physical, but that it has to have one thing it sells to many people, rather than doing custom work for individual clients.
Open source is inherently not a product -- at least, not in the sense of being something you can sell to many people. The frequent cry of "but we'll make money selling support" ignores a critical fact: There aren't enough hours in the day to get rich by selling support contracts.
The one "open source" company which was successful at making lots of money -- MySQL -- wasn't in fact an open source company: They made their money selling a commercial product, and they just happened to use releasing a GPLed version as a cheap advertising mechanism.
I strongly support open source software; but a startup which builds open source software? It ain't a startup.