Oh no, I knew it wasn't planned by NASA. I didn't know he only thought of it a few hours before... and now that I think about it, I'm not sure I believe that. Thanks for posting all the info, ptero... Unfortunately I kept getting "oops, broken link" messages. (plus, I think you might have left a link open--your second paragraph is hot...)
Plenty of info about this issue at these pages: 1, 2, 3, 4, 5, 6.
Reading this new info, I'm surprised to learn that Armstrong came up with the line himself, and that he did so only a few hours before he spoke it. I'd always assumed that the historic line was planned well in advance by NASA.
Well, some people say he DID say the "a," only it's hard to tell on the recording, what with the sound quality. It's believable that he did, at any rate. And like you said, it makes little sense without it.
I remember Buzz Aldrin, after jumping off the ladder behind Armstrong, said something like, "I don't know what Neil was talking about, that was a pretty big step after all." :)
Chained_bear, I love that you included the missing "a". You know, the sentence makes no sense without it, but yet most people don't even notice that it's missing. I guess we're just so used to hearing it without the "a" that we've gotten used to the awkward construction.
I know it's all Neil Armstrong's fault for flubbing his line, but I don't blame him. He was understandably distracted at the time. :-)