Comment 1 for bug 1339380

Actually there shouldn't be a big rupture visually, as lightdm-gtk-greeter uses the user's wallpaper (at least if the service that sets the wallpaper in the session supports accountsservice, e.g. in Xubuntu that is the case). In Xubuntu the transition is that the greeter disappears and the elements of the desktop appear with the wallpaper remaining the same.

Not sure where you'd wanna implement a fade there. Theoretically the greeter could do a fade-to-black, but that might not even be desirable (see the transition in Xubuntu described above) and the result depends much on what the session does. A cross-fade isn't really doable as far as I can tell.