The Developer Edition of the XPS 13 comes with Ubuntu 12.04 preinstalled. However, if you ever need to reinstall (and can't use your own USB image), the original ISO is here:
http://hwe.ubuntu.com/uds-q/dellxps/amd64/sputnik-precise-amd64.iso
To install it, you'll need to use the instructions available on the USB Installation Instructions page on help.ubuntu.com
Ubuntu is clearly stressing that this isn't an officially supported release:
Text Snippet:
This image is derived from the Ubuntu 12.04 (Precise) Release:
* this image is not considered an official Ubuntu release
* this image is not officially supported
* this image is for demo purposes only
This post is more of a bookmark for me personally, but I guess I'm not the only one whose bought this sweet piece of hardware.