My TV will do 1080p from a capable source via one of it's HDMI hookups. I have the TV hooked up to my Motorola Cable box via HDMI. Cox Communications is my Cable Provider and their HD Box came pre-set to output "720p and Keep 4:3 image - 480i".
HD stations all come out as 720p when uisng the HDMI from cable box to my TV. However, when I select the TV Input (NON-HDMI) on my TV and watch a local HD station via the TV's tuner, it will output the local HD stations in their native format i.e. some are 1080i and some are 720p (as you mentioned ABC being one of them). Unfortunately signals coming through from the Cable Box via HDMI Input on the TV will make all stations 720p regardless of their original output.