Philip,
OpenACS' intention is to bring the butt-kicking ACS to a completely open source environment. That means porting it to Free (speech) databases such as PostgreSQL and Interbase.
How viable a community OpenACS is? Just look at the download numbers... thousands in every release. Hundreds of postings in our bboards. Lots of bug reports and community assistance in porting the code. I'd say it's a pretty good community.
On "aD doesn't care about OpenACS" issue, I don't think it is that way. I met with Adam Farkas (Developer Relations for aD) in September and we had long talks about ACS 4 and OpenACS and aD is supporting it and making moves toward making things better and easier for us. Look for postings by Adam here at this bboard and you'll find that more than once he requested OpenACS developers and users to give feedback on ACS 4's design prior to release.
I agree with Dan that porting is much easier than developing. The problems that ACS is out to solve are not trivial, and there are dozens of bloated, hyped, and/or closed-source similars out there. I am glad that we have an enterprise-level piece of software to port.