The basic steps for loading a Raspberry Pi image after downloading and unzipping the image (to /dev/sdg1 for example) are:

sudo umount /dev/sdg1
sudo dd bs=4M if=2013-09-10-wheezy-raspbian.img of=/dev/sdg1
sudo sync

This didn’t work for me. It seemed to write the data but left the card unbootable by the RPi. In addition, I couldn’t mount and view the contents in Ubuntu either. Not sure why, though there’s probably something obvious I’m missing. Instead, I found a lazier solution: using the Ubuntu Disks graphical tool. Find your drive in the device list, stop it if necessary, and then use the “Restore Disk Image…” option from the actions cog button.