Titles are automatically titlecased when submitted but not when edited. The workaround is to submit and immediately edit afterwards.

Unfortunately many blogs have their titles all uppercase instead of using small caps. For me the use case where I copy an all uppercase title and want it to be titlecased on HN is much more common than the case where I actually want a string of caps. I'm quite ok with the current behavior.

weinzierl got it right. We've edited the title back.