Nope, none that I know of. I've seen the same thing. Only if you have a scheduled appointment which is an "All Day" event will it show in BOLD. If not, it will just show a little dot / line for however long you have the appointment scheduled for on that day.
Are you sure yours doesn't. I just checked, set a call reminder in there and it shows up as bold for today, have no other events set. I didn't change any settings from what it was when I got it.