Over-the-air HD is capable of carrying up to Dolby Digital 5.1, but it depends on the source material (generally the only times I would expect to see a DD 5.1 signal would be for prime time broadcasts, and even then it's not necessarily a sure bet) and it requires that the broadcasting station's hardware supports DD 5.1. I just ran through all my OTA HD channels, and at the moment (6:30 in the morning) they're all broadcasting in DD 2.0. It's the same for the HD stations delivered by my cable provider. TNT-HD and Discovery HD both broadcast in 5.1 almost all the time, but they're not available OTA.

Basically, what you're running into is pretty common. You may want to try some recently-produced prime time material and see if they're broadcasting that with DD 5.1. For the cases where it's DD 2.0, though, I end up apply Pro Logic IIx to get surround.