Convert the .iso file to .img using the convert option of hdiutil

1	hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso

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)
Execute the following:

1	sudo dd if=/path/to/downloaded.img of=/dev/rdiskN 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 the ‘Disk Utility.app’ and unmount (don’t eject) the drive.
Run diskutil eject /dev/diskN and remove your flash media when the command completes

You now have a bootable USB drive to install ubuntu where ever you please.