I cooked up a patch that allows to setup a multisite installation on localhost, albeit only for subfolder installations.
A possibly related niggle that I ran into while installing a test site was #12459. If your database prefix is empty as well, you’ll need that patch as well.
The primary purpose of these two is, obviously, to allow developers to use the MS functionality without needing to constantly go back and forth with a server using SSH or FTP.
Please give both patches some testing so we can get both checked in before we’re in beta. Doing so will give the new MS functionality a lot more exposure to testers.