You may recall that a Little Orphan Annie decoder pin story became a key plot element in the movie, The Christmas Story.
The code here was fairly easy to crack, with the alphabet simply offset by a certain number of letters; in this case it's 3 letters, so that a D should be read as A, E as B, etc. A, B and C are X, Y and Z, respectively. So decoding the entire message in this issue reveals the following:
This summer we will publish a giant Superman annual featuring stories of the planet Krypton.
And, as promised:
Two months later, in Action #289:
In this one, the alphabet is offset by 5 letters, so that F is A, G is B, etc.
Ultra-Boy is the newest member of the Legion of Super-Heroes. See the July issue of Superboy.
The messages were not always advertisements of coming attractions; they were often inspirational statements about doing your schoolwork, or getting plenty of fresh air and exercise. For example, from Superman #68 comes this tip: "Keep healthy, cultivate a sense of humor, and learn to see yourself as others see you."
As far as I can tell, the planets mentioned at the beginning of the code have no real meaning.
Update: Snard in the comments points to this scan of the actual code key. Anonymous notes that the planets correspond to the numbers (Mercury 1, Venus 2, Mars 3) in terms of their ranking in terms of proximity to the sun, with the rather odd exception that Earth is omitted and Krypton becomes number 9.