It seems like they should honor the deal for a specified period of time. Most Amazon SO's come with an expiration date, so should this one.
As far the $5 video deal, meh. Amazon Instant Video is a huge rip off IMO. They want a king's ransom for just one movie. You can get a whole month worth of rentals at Netflix, Hulu, etc. for less than the price to rent just two AIV's.