As you have already found out, the mountpoints are in /run/user/$USER/gvfs/ (or /var/run/user/$UID/gvfs) and are named after the protocol, connection type and address they use. Which makes things more difficult, because the connection address may change every time you replug the device, even if it is the same port. If you have the same device twice it gets even worse.

You can use lsusb to display all connected USB devices from terminal:

$ lsusb | grep Google   # Note: This is a Nexus 4, change accordingly

Bus 002 Device 025: ID 18d1:4ee2 Google Inc.

As you have noticed Nautilus also gives this information via tool tip. You will find the encoded form of e.g. mtp://[usb:002,025] in /run/user/$USER/gvfs (or /var/run/user/$UID/gvfs) as mtp:host=%5Busb%3A002%2C025%5D

Asker's edit: This seems to require a newer version of libmtp and/or gvfs than is available per default in 13.04. Run sudo add-apt-repository ppa:langdalepl/gvfs-mtp and update before doing anything else.

Edit: PPA not needed anymore in saucy/13.10 and newer, filenames are listed in terminal as they are in Nautilus.

Edit 2016-01-11: I removed the script that was previously included in this answer due to lack of time for maintenance and improvements. You can still find it in the revision history.