How do I create an Ubuntu live USB using a Mac?

Taken from here:

We would encourage Mac users to download Ubuntu Desktop Edition by burning a CD. But if you prefer to use a USB stick, please follow the instructions below.

Note: this procedure requires that you create an .img file from the .iso file you download. It will also change the filesystem that is on the USB stick to make it bootable, so backup all data before continuing.

Tip: Drag and drop a file from Finder to Terminal to 'paste' the full path without risking typing errors.

1. Download Ubuntu Desktop

2. Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight)

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

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

   Note: OS X tends to put the .dmg ending on the output file automatically.

4. Run diskutil list to get the current list of devices

5. Insert your flash media

6. Run diskutil list again and determine the device node assigned to your flash media
   (e.g. /dev/disk2)

7. Run

   diskutil unmountDisk /dev/diskN
   

   (replace N with the disk number from the last command; in the previous example, N would be 2)

8. Execute the following command while replacing /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg).

   sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m
   

   • 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:/devdiskN: Resource busy, make sure the disk is not in use. Start the Disk Utility.app and unmount (don't eject) the drive.

9. Run diskutil eject /dev/diskN and remove your flash media when the command completes

10. Restart your Mac and press Alt while the Mac is restarting to choose the USB-Stick

You can do it really easily with unetbootin which is available for Mac OS - the upside being it doesn't need more than a few clicks on a simple GUI.

(note this is only for newer Intel Mac's, and won't work with older PowerPC machines unfortunately.)