Mike,
That's a very valuable $0.02. Thanks!
I think you are right and that the number of released projects is a better criteria. (And we're not in a hurry.) We definitely need more experience with the whole process.
I'm going to type a couple of related thoughts that came to mind when I read your response, just to get them out there for everyone, while I have a few more minutes.
We discussed the maturity of our projects a bit during the committee meeting. One of the interesting things about our projects in Science (ICE, DAWNSci, Chemclipse) is that even though they are currently incubating they are pretty large, established and mature from our pre-Eclipse days. The trip through incubation for these projects is more about compliance with IP and PMC needs than development of code, although that is still happening. I think it is safe to say that we what we are really incubating with this three projects is the neither the technology nor the governance, but the IP trail and membership in the project community, or at least it seems so to me. It looks like the same may be true for some possible future projects as well, although the viz project would be close to a completely new effort.
Another thing we discussed is that science projects typically have strong relationships with the government of the country where they were written. This presents a whole different set of issues than, say, pure technology projects. Help with dealing with these issues could be one really great benefit of a Science PMC.