Comment 9 for bug 1267742

The fragments you can see after the greeter exits and the session starts is the root-pixmap, which is shown so that there's no flickering until (in Xubuntu's case) xfdesktop/xfwm4 starts.
However, in the case of nouveau, it doesn't seem to make matters any better.

The greeter bug is fixed then, thanks for confirming/testing!