I have an album (hostedimages) where I put photos used in message forums and the like. Therefore I've always allowed hotlinking since that allows me to just cut/paste the url into the BBCode. Suddenly this has stopped working for any NEW images that I put there. I can still see images in any of the old posts I've made on those forums.

I'd don't see any changes in the zp-admin control panel; image protection is still set to "unprotected". What else should I look for?

The hotlink protection option is only for full image served via the theme and itis always bypassed if you link to the images on the filesystem directly because Zenphoto is not involved then.

You should look at your Zenphoto and server logs for any related error message.