This has always been a feature. If you edit your post within 5 minutes it doesn't count as an edit.
Jeff and Joel have discussed this in an early one of their weekly podcasts.
Jeff wanted this because he would go and edit his own stuff.
It was podcast number 20. You can also read the transcription;
I also try to copy a lot of things I've seen online that have been successful, like conventions. Let me give you a specific example: PHP-BB, and I'm sure there are other web discussion boards that do this too but PHP-BB is the one I know, has this editing convention. When you post you can edit your own posts, and I noticed that when people are using PHP-BB that right after you post something you'll always notice some goofy mistake that you made, like immediately. This happens to me 9 times out of ten I'll post and think "oh, I should have talked about this" or "I missed that word," so you immediately go in and edit. At a certain threshold these are not treated as real edits, they're treated as just going back in time to pretend that it is the post you originally made. It doesn't kick off the whole auditing trail of you having edited it 50 times. One of the first things we did in StackOverflow is actually implement that. I remember talking to Geoff Dalgas about that, and he's like "why do we have to have this?" I said, "you don't understand, this feature has to be in there on day 1 otherwise we're going to have so many revisions that are just in the first minute or two after posting and are just silly little things that are being corrected."
You not actually recording the revision? You're not doing the diff thing?
[Not] within the threshold. Right now the threshold is actually 5 minutes. So up to 5 minutes after you post, if you edit your own stuff. Now if I go edit it, it's a real revision.