How are new Stack Overflow mini-sites created?
Personally, I would like to see this rolled out. I think several sites have logical groupings. For example, on Stack Overflow, being able to have a view for .NET (platform, as well as C#, F#, VB, ASP.NET, ASP.NET MVC, SQL Server, Visual Studio, Boo...), the Java platform (Java, Scala, Clojure, JRuby, Jython...), database development (MySQL, MSSQL, PostgreSQL, sqlite, mongodb...).
On other sites, I see this being useful too. For example, on a site like Gaming or SciFi where there are series being discussed, you don't need a "catch all" tag to make it easy to sort or view. Someone interested in all things Stargate could go to stargate.scifi.stackexchange.com and have a view of things tagged stargate, stargate-atlantis, stargate-sg1, and stargate-universe easily and cleanly, without any chance.
The big drawing point comes in with large subcommunities, though. I see this idea being used more on Programmers and maybe Project Management (when it launches). The Agile development community is, in its own right, its own community. They are Programmers and Project Managers, but they have their own interests. In fact, they wanted their own Stack Exchange site, but it was too overlapping with P.SE and PM.SE. If there are groups that want their own Stack Exchange, but it overlaps, just make it a sub-Stack Exchange of an existing site (so, in this example, agile.programmers.stackexchange.com could be used to catch all questions tagged agile, scrum, extreme-programming, dsdm, kanban, lean, and so on).
Just for a note...these are just a few examples. It would be up to each site to identify ones that actually work. I would suspect that determining which ones would be valid would be a combination of looking at what people have as favorite tags, what tags people view (something like how frequented tags works), looking at overlapping proposals for other Stack Exchange sites (especially those closed for being too close to an existing site), and feedback from the site's Meta as to which groupings would be best to ensure the mini sites actually work and would be used.
Now that it's technically possible to do, when might it be rolled out? Or is this not even on the table yet?