Today is my first day on SilverStripe, I'm evaluating it for a project I've got coming up. So anything that I do is probably wrong!
Even though adding the genericdataadmin module made the menu item appear on the backend, it appeared that the functionality mentioned in the wiki just wasn't there - I could create subsites, but the drop down list did not appear on the Site Content page and if I tried accessing the subdomain on the normal site it would give an error message. I suspect that it will need a more experienced mind than mine to get this working with 2.3.0.
It even says at the top of the genericadminmodule:
* @deprecated Use [email protected] ModelAdmin} instead, it's much more advanced and does much more for you.
Subsites are a requirement for my project, but not an urgent one, so I may still be able to use SilverStripe (it's certainly the best CMS I've seen so far), but I'll be waiting with baited breath for subsites to be fixed for 2.3.0.