One way to know that the open source market is maturing is to analyze how fast companies are able to get to significant revenues. By "significant" I mean $10M and on a sharp, upward ramp. When I started in the business of open source (2000, with Lineo), it was horribly difficult to pull in $1M in revenues, much less $10M.
Now? The ink will still be dry on your VC term sheets when you cross that threshold. Really.