The link must have been specific to me, for when you click on it you are taken to a page that says you've already completed the form. I tried clicking around edmunds.com again today and couldn't get the offer to pop up. It didn't take long to pop up on Friday -- I had just clicked on "New Cars" and suddenly there it was at the top. Earlier in the thread, somebody tried to link to the specific rebate offer, but couldn't.
FWIW, here's a link to the Terms and Conditions of the offer:
Nissan Altima Special Offer
The rebate seems to be elusive. However, seeing as how it looked like it had expired weeks ago, only to surface again last Friday, I'd say it's possible it will show up again. My advice would be to keep checking every day and/or email the address in the Terms and Conditions.