I've been having this problem ever since I started using WordPress this past April.
Certain posts go up with dates missing. The author name is there, but the date is mysteriously absent. It generally affects about 25% of posts. And it sometimes happens to posts when they get bumped down by a new post. Very very odd.
The best way to see what I'm talking about would just be to visit the front page. You'll see what I'm talking about right away as you scroll down.
If anybody has any information that could help me out here, I would greatly appreciate it.