You can read more about WHYY's digital broadcasts at this link:http://www.whyy.org/tv12/dtv/index.html
That page also includes a .pdf (Acrobat) file with the current month's schedule of digital programming. 12-1 simulcasts the WHYY analog channel's programming in the morning and afternoon, and then they switch from their standard-definition digital signal to their high-definition digital signal in the evening. Whenever there is no special program scheduled to be broadcast in the evening (see schedule), they run a continuous cycle of short documentary films produced in HDTV as a "demo loop." It's tedious stuff after you have seen it a few times, but it's a great reliable way for us to show off our HDTV tuners to visitors!
One big advantage to digital broadcasting is that the station controlling the channel can split the signal into sub channels. 12-4 is one of those subchannels. (Why it is not 12-2 or 12-3 is a mystery to me.) I actually saw content broadcast on 12-4 when I first got my HDTV tuner. It was probably on November 18 or 19, and the show was some medical education program.
WHYY can potentially lease time on that subchannel to corporate or institutional entities to earn some extra $$ for the station. I've read elsewhere that some PBS digital stations use a subchannel to broadcast PBS Kids shows on a continuous cycle.
I have also not yet gotten a signal on 67 for WCAU (NBC). I realize that the November 30 date must have been a goal rather than a deadline. I hope if anyone here is on friendly terms with the good people at WCAU, you might ask about the status of their new broadcast facility. I for one am looking forward to it greatly!