This is just a post which serves as a mental note to me, but others might find it useful as well. The command to create an USB install disk for OS X is as follows:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled/ --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
This is considering your USB disk is mounted as /Volumes/Untitled and you installed the OS X Yosemite dmg in /Applications.
Please note that if you select the wrong disk you might overwrite your current installation!
The output of the command should look like this:
Password:
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying boot files...
Copy complete.
Done.