I've wondered about all these new sites for a while; Is it just someone who paid to use Stack Exchange, or is it just a new addition to a wholly owned collection?
It's part of the third phase of the process that produced Stack Overflow.
First, Jeff Atwood and Joel Spolsky had an idea for a site called Stack Overflow, and implemented it. They then added a few other sites.
Second, they introduced Stack Exchange (now often called SE 1.0), where people could pay for Stack Overflow-type sites. Apparently, quite a few people did, and some sites (like Math Overflow) were highly successful.
Third, they dropped the SE 1.0 model and went to a new idea for Stack Exchange. In this, people would make proposals in a new site called Area 51, and if they seemed to have enough support they'd make a real site of it. This is one of those. No money changes hands, but since the SO team makes money off SO and the other sites (presumably through advertising), they hope to do the same here.
(In addition, some other people liked the SO idea, and independently created sites much like SO, but that's not quite relevant here.)
So, this is indeed owned and run by Jeff and Joel et al. Currently, it's in public beta, which means they're seeing how it works out before committing to it for real. They, not us, make the decision as to when it goes truly live or is just dumped as a failed site.
To be specific, Programmers is a site that's being closely watched. People on Meta Stack Overflow are concerned that it isn't turning out well, and you can go there if you want to read about it.
ChrisF answered this, but I simply wish to expand on it a bit more than would be suitable as a comment.
Stack Overflow was started as a project by Jeff Atwood , which he tried to move into different marketable directions. The Stack Exchange group of sites uses the same engine developed on SO, and has been growing into the network of sites you now see.
Stack Exchange is likely in it's late infancy, as we're seeing sites exit their beta phases into full fledged websites. The creation of area 51 is a place where new sites can be 'born' into the network.
If you didn't get to it from the stack exchange site, it's not part of the network. If you link together your account with another SE site, it should notify you they are connected under "accounts" on your profile, because it centralizes the oAuth part, which is really neat because we can do cross-site notification stuff.