Looking around other cards on the board, I can't actually see any user comments, just a note stating that a user commented on the card, as shown below
Here is a view from another card with many more missing comments
I'm using Chrome 22.0.1229.79 m, which it reports as being up-to-date, however a colleague of mine doesn't see the same problem using IE 9. I've just checked in IE9.0.8112.16421, and it works fine for me too.
Is this a known problem? I can't find any other reports of this problem?