Note: this procedure requires an .img file that you will be required to create from the .iso file you download.
Download the desired file. Convert the .iso file to .img using the convert option of hdiutil
hdiutil convert /path/to/ubuntu.iso -format UDRW -o /path/to/target.img
Note: OS X tends to put the .dmg ending on the output file automatically. Run
diskutil list
to get the current list of devices. Insert your flash media. Run
diskutil list
again and determine the device node assigned to your flash media (e.g. /dev/disk2). Run
diskutil unmountDisk /dev/diskN
(replace N with the disk number from the last command; in the previous example, N would be 2)
If you see the error "Unmount of diskN failed: at least one volume could not be unmounted", start Disk Utility.app and unmount the volume (don't eject).
Execute
sudo dd if=/path/to/downloaded.img of=/dev/diskN bs=1m
(replace /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg).
Using /dev/rdisk instead of /dev/disk may be faster.
If you see the error dd: Invalid number '1m', you are using GNU dd. Use the same command but replace bs=1m with bs=1M.
If you see the error dd: /dev/diskN: Resource busy, make sure the disk is not in use. Start Disk Utility.app and unmount the volume (don't eject). Run
diskutil eject /dev/diskN
and remove your flash media when the command completes
Restart your Mac and press alt while the Mac is restarting to choose the USB-Stick