I used the following commands to create a bootable Ubuntu thumbdrive using OS X.

First, I ran the following command to find my thumbdrive after connecting it to my computer.

diskutil list  

In this case, my thumbdrive was /dev/disk1

Next, I converted the Ubuntu disk image from .iso to .img using hdiutil

hdiutil convert -format UDRW -o ~/Downloads/ubuntu-12.04.3-desktop-amd64.img ~/Downloads/ubuntu-12.04.3-desktop-amd64.iso  

I unmounted the thumbdrive

diskutil unmountDisk /dev/disk1  

and then wrote the image to the thumbdrive

sudo dd if=~/Downloads/ubuntu-12.04.3-desktop-amd64.img of=/dev/rdisk1 bs=1m  

After it was finished writing, I ejected the thumbdrive

diskutil eject /dev/disk1  

After this, I was able to plug the drive into a laptop and boot into Ubuntu with no issues.