I think the key may be that it isn't so much the HD but ATSC. Ie I may be wrong, but I think that the HD receivers that do AC3 are all the ones that also have a built in ATSC capability. Since the OTA ATSC channels all require AC3 decoding, those receivers have it built in, however an HD receiver that doesn't have ATSC doesn't necessarily have AC3 built in.
The Quali-TV doesn't have a OTA tuner in it
That fits what I said perfectly. Ie if it does have ATSC, then it must have AC3 capability, but if it doesn't have ATSC, then it may have AC3 but probably won't, because receiver manufacturers seem to have decided that it's not worth the extra cost for some reason. I can't think of a receiver, either SD or HD, that doesn't have ATSC but DOES have AC3 built in. And I can't think of a receiver that does have ATSC that doesn't have AC3. So I still say that at least now, ATSC is the key to whether a receiver will do AC3.