Hey jam13,
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 {@link 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.
tt