I have a really peculiar problem. Comments will show up in counts, but won't be displayed. E.g. the Home page says "2 comments" (and I can see them approved in admin), but the post page display none. However, pingbacks are counted and displayed correctly.
I switched between themes, including the default theme. The problem persists.
I'm using version 2.3.3. For fear of losing more comments in upgrading to 2.5.1 (a few bug report on that), I haven't upgrade yet.