Is there a keyboard shortcut to access the gnome-shell messaging tray? E.g. For responding to incoming IM, etc.
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.
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.