If you just got your Surface RT recently (let's just say starting December 5, 2012...) and try to run windows update you might get an error with the cryptic code 8024004C. As of typing this, it's 'unreported' in Google or Bing. I got it and was initially worried but I looked at the list of updates and saw the Firmware update in the list of scheduled updates. I unchecked the Firmware update and kept the optional update of Microsoft Office unchecked, Re-Tried updating and the updates worked. I am assuming the Firmware update needs some of the other updates to be in place. The updates are still going on. Will update this post, when I am able to install the firmware update!
Okay, Windows RT updated itself with the other items first, then installed the firmware during reboot, so I didn't have to go back and install the update manually. The only optional update remaining was Office RT update. That's a 500+ MB download, currently in progress! BTW Windows sets you up for auto-installation of updates. If that's not your thing do change it from Control Panel!
As pointed out by Joey in the comments below, it is more likely that your RT won't install firmware updates if you don't have enough juice. I clearly remember my Surface being on charge when I was trying to update. So thanks Joey, that's a very useful remark.