In GNOME 3.8 you can switch workspace by using the Super + Page Up or Super + Page Down. Unfortunately you can do the same with Ctrl + Alt + Arrow Up or Ctrl + Alt + Arrow Down. The last one conflicts with a neat shortcut in Eclipse to duplicate lines.
Under Settings > Keyboard you will find the hotkey to switch workspaces. The Ctrl + Alt Arrow Up/Down is always bound and is not shown.
Check which keys are bound here:
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-up ['<Super>Page_Up', '<Control><Alt>Up'] gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-down ['<Super>Page_Down', '<Control><Alt>Down']
You can clearly see that both commands have 2 hotkeys assigned.
To remove the Ctrl + Alt variant run this:
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-down '["<Super>Page_Down"]' gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-up '["<Super>Page_Up"]'
The hotkeys Super + Page Up and Super + Page Down are retained but Ctrl + Alt + Arrow Up and Ctrl + Alt + Arrow Down are removed.