I don't think GDM 3.0 supports an on screen keyboard; you'll need to use LightDM (which is the new Ubuntu default as it's far more powerful for our use cases & it's supposed to be more themable too). For Ubuntu 12.04, GDM 3.2 or 3.4 will probably have the GNOME Shell screen keyboard built-in.
I don't think GDM 3.0 supports an on screen keyboard; you'll need to use LightDM (which is the new Ubuntu default as it's far more powerful for our use cases & it's supposed to be more themable too). For Ubuntu 12.04, GDM 3.2 or 3.4 will probably have the GNOME Shell screen keyboard built-in.