I haven't received either confirmation of delivery, or my letter returned as undeliverable. In my experience, this is an unusually slow pace for delivery confirmation mail, but I have never had a delivery failure with that system. I suspect that my letter probably had to sit in the Atlanta, Georgia post office for the duration of the time the addressee is given to pick it up following a failed attempt at delivery, and this will be my first "undeliverable" notice.
Checking on the CDC's FOIA status request page, however, does show that there's been a change. I've been upgraded from "Pending Program Search."
So that's it - not a huge update, just a quick note, and a little hope.