Just like XFCE did a week ago, GNOME will move from /opt/gnome to /usr. Together with this move, one package becomes completely obsolete: gnome-common. This package is only required for packages in /opt/gnome to stay functional. The GNOME environment in /usr won't depend on it anymore.
For those who don't want to make the switch in one run (the upgrade includes +/- 150 packages, worth several megabytes), the upgrade procedure tries not to break these things. During the process of creating these packages, my test system was a mix of /opt/gnome and /usr packages, which works fine. Keep in mind that for these applications gnome-common will be required to operate. This is no longer a dependency however.
Due to the changed paths some downsides will happen: profiles will have all gnome-panel launchers pointing to /opt/gnome. These launchers can't be loaded by the gnome panel in some cases, causing them to get lost. These launchers should be manually recreated.