This offer is for those with the Kindle Wi-Fi with Special Offers or Kindle 3G + Wi-Fi with Special Offers only. View the Special Offers on your Kindle (Menu, Special Offers) and you should see a new promotion (if you don't, turn on your wireless, click Menu, Sync & Check for New Items, then after the sync has completed, go to the Special Offers screen):
50% off a Kindle Lighted Leather Cover
Click on offer, then click on the link on the offer page sign up and get an email with the promotion code. Sign-up for this offer expires on May 28 (so those who have just ordered a KSO have plenty of time to take advantage of this deal).
You'll get an email (right away), a link to Kindle Lighted Leather Covers page and a promotion code to enter at checkout. Once you have the promotional code, you have until Aug 28 to complete your purchase. Details and terms are on this page. Like previous offers, this one requires you to use the full checkout process in order to enter your promotional code. Also, like all Amazon sales that use promotional codes, if you have a gift card balance, you must use it for the payment (if there is not a sufficient balance, then you can pick which credit card or other payment to use).
Maybe now I'll order a cover for my naked KSO, to protect it a bit better in the event it gets knocked off the shelf.