gconftool-2 --type bool --set /apps/nautilus/preferences/media_automount false
The devices will still appear in computer:/// (in nautilus) where you can right click on them to select the mount action if and when you want them to be mounted. If and when you want to re-enable to auto-mounting feature execute:
gconftool-2 --type bool --set /apps/nautilus/preferences/media_automount true