I would say its an IE bug, though I haven’t heard of it before.Changing “small” to .8em or even “smaller” instantly clears up your menu. In fact, IE is not honoring your use of small in any class, you just might not have noticed it. Your blog entries themselves are much larger on IE due to the same problem.
On the calendar I do notice that IE is allowing the div’s in your right column to be different sizes while Mozilla is forcing them all to be the >same size. This is no doubt the real source of difference in your calendar. What is really strange is that the issue clears up if you remove this line from .rightnav: