It happened and I almost didn't notice.
I switched writing my Level 1 and Level 2 headings from title case to sentence case:
And, as you can see in the image above, it's not just the titles, it's also the captions!
Why make the switch? Because title case headings were starting to look a little old fashioned to me. And, having learnt so much about typography over the last year or two, I now understand that title case is a crutch I no longer need to rely on. I can use good typography to help readers distinguish headings from body text. And good typography plus good design do a much more effective job that merely capitalizing the important words in a heading ever did.
Actually, this passage from Wikipedia's entry for 'Letter case' sums this up nicely:
So there you go.