I ran into this site: http://www.semanticoverflow.com/
But I can't find it on here: http://stackexchange.com/sites
Is it even an "official" Stack Exchange project?
According to the headers
it is a Stack Exchange 1.0 site. This service was discontinued and replaced with Area 51 and Stack Exchange 2.0, and since it doesn't have the Stack Exchange Multi Colider Thingy Me BopTM, it is not an official Stack Exchange site, albeit running the older Stack Exchange software. For the site to become part of the Stack Exchange network, it will need to be added to Area 51 and follow the complete process.
I suspect this is one of the early sites that was launched prior to the establishment of Stack Exchange, as it's definitely running an (albeit older) version of the software. From what I can recall, it was possible to pay for the hosting of a stack site for a while, and I'm guessing you're looking at an example of this.
It looks like it launched ~October, 2009, which would make sense in this context.
There's some launch blurb which looks like it's form one of the creators (www.SemanticOverflow.com – the Web 2.0 Q&A site for all things Web 3.0.) which also backs up this theory.