After upgrading to Xfce 4.10 from 4.8, I noticed that the clock/notification area no longer pushed to the right edge of the panel.
This is due to a change in the Window Buttons panel plugin which no longer expands to fill the available space[1].
To emulate the old behavior, add a transparent separator between the window buttons and the clock/notification area, setting its expand property.
Voilla!
