Access gnome-shell messaging tray using keyboard
Gnome 3.6 and up
This extension has not yet been updated for Gnome 3.6 But the functionality it provided for the message tray is now a built in feature of Gnome 3.6.
Gnome 3.4 and earlier
A GNOME extension has been developed to enable keyboard access to messaging tray.
- https://extensions.gnome.org/extension/313/helpful-keybindings/
You can, by default, use Super + M to display the messaging tray and Super + Space to activate the messaging tray. You can also configure the keyboard shortcut to your wish.
This is a built in feature of Gnome 3.6's Message Tray.
You can customise the shortcut to open the tray, and then use arrow keys to navigate the various notifications.