I have a XPS12 with the same issue on the login-screen (the brightness is set to level 0 -> backlight completely turned off). However my backlight buttons work so that I can adjust the brightness correctly, but it is still annoying.

I wish I could at least adjust it manually but, for me, it's still not working after today's updates.

UPDATE: This issue was confirmed by Dell and they are working on a fix. Please follow this bug report for further information: bugs.launchpad.net/.../1169376

Did you try shift+Fn+backlight-brightness? I have the XPS12 where the backlight-adjustment works, but I noticed that  this shortcut is working as well, although it seems not to be registered by Ubuntu (brightness notification didn't change).

shift+Fn+backlight-brightness also allows me smaller steps (~2%)

Just got a new Kernel this morning, 3.8.0-27 and no luck on the backlight adjustment working still.

Are there really this many problems with updates or is it not that bad? I hesitate to buy this...as a developer, I love to have the latest release of a Ubuntu version..but how well is it supported by Dell? I expected that before it comes out, the usual things should work like brightness setting and stuff...for Ubuntu this is a major fail, having a specific Ubuntu ultrabook and then still things break after updating/upgrading...

Also, @boylenssen,  the pre-loaded XPS13 running Ubuntu 12.04 has working backlight controls.  

Here's the latest on the backlight problems:

A quirk patch for all XPS 13 models has now been merged into the upstream mainline Linux tree.  There were some ACPI issues upstream that were hampering some of the efforts fo fix this and they have since been reverted.  As of Linux kernel version "3.11-rc3", backlight control works on

all three XPS13 models (L321X, L322X HD, and L322X Full-HD).  

The backlight problems only affected the Full-HD models, which explains why we were getting mixed results from the field as to whether or not 13.04 was actually broken.

So its finally fixed upstream, and hopefully it will stay that way.

Ubuntu Saucy (13.10) will pick up the fix automatically as soon as it moves to the mainline v3.11 kernel baseline (probably soon, but some time well in advance of the Saucy release in any case).

The fix has also been tagged stable-backport kernels, so it may get picked up by the maintainers of the upstream stable v3.10 and v3.8 trees.  It should appear in Linux v3.8.13.6 when that gets released later this week.

Ubuntu Raring's (13.04) v3.8-based kernel is currently carrying an earlier version of the quirk (which only works for non-FHD models), but our maintainer will ensure that Raring picks up the all-models fix in its next update (or the one after, depending on scheduling).

So while there are no exact ETA's on when the backlight will finally be fixed for 13.04 and higher, we can say that it should be very soon.  

Entering this command worked for me: echo 0 | sudo tee /sys/class/backlight/intel_backlight/brightness

I had upgraded to Ubuntu 13.04 and the backlight stopped working. After I upgraded my graphics driver it now works without any additionally tweaking. I got it from here http://01.org/