SUMMARY: My hosting service does it again.
I just realized that it isn't that no one has been posting comments on my blogs--it's that I haven't been getting email notification about them. Along with the "upgrade" that hosed my blog access a couple of weeks back, they also neglected to convert my list of email aliases, including the one for comment notification.
Sooo--I've fixed that manually, and I just ran down all the December posts looking for comments that I've missed, and responded to some. Sorry about the delay!