I'm sure most of the "usual suspects" will show up here, but Rick arrived early and he and David make the case. If there was a moment of critical mass it was as Rick described. When Earl Scruggs banjo joined, that was it. I don't think there are many forms of music where you can actually trace the origin to a particular moment in time, but Bluegrass is one.
Here's a PREVIOUS THREAD that also talks about it and makes for interesting reading too.