I think I figured it out...
Sometime within the last 24 hrs, WordPress.com is now requiring that visitors login in order to comment using their gravatars (previously, you could just paste your gravatar email into the comment form). Phony and non-gravatar email addys still work in the comment form like they used to.
I assume this was done to prevent nic-jacking or stealing/borrowing others' gravatars, or for some related security concern.
This is actually kind of a bummer; for our site, we have email addys for gravatars posted in our sidebar that are themed, and we encourage folks who don't have gravatars to use them. Now, they're pretty much worthless.
Or for me, I like to use different gravatars for different situations. From here on out, it will require a lot of extra logging in, then out, then in again.
In any case, some sort of announcement would have been nice.