Life's tough.my finger is getting a workout
Different channels may be broadcasting at different resolutions. E.g., see http://en.wikipedia.org/wiki/High-definition_television_in_the_United_States Also, some channels may elect to transmit in SD (such as some of the retro TV stations like AntennaTV, MeTV or THiS TV, some religious stations) and the digital subchannels are likely also SD.
Also, even when the channel itself is normally HD with a 16:9 picture, there may be some commercials, older shows, or older made-for-TV movies that have the 4:3 aspect ratio. (And coming up soon is the famous Judy Garland in The Wizard of Oz, which was filmed in 1.37:1, which is just slightly wider than 4:3 or 1.33:1, but far narrower than 16:9 or 1.78:1.)
So, while watching a HD channel, it is common to see some content switch to 4:3 (black pillars on the sides of the picture) and then switch back to filling the full 16:9 screen (or, if a widescreen movie, may even see the top and bottom black bars from showing the movie in letterbox).
Complicating that is how the TV reacts when presented a change in screen mode, a second or two while the TV detects and adjusts to the new resolution is normal.
I have just one TV connected to cable (see my "Bedroom" list in my signature), and that through a HD DVR that I am leasing from Comcast. To avoid the delays while the TV adjust to various resolutions, I configured the HD DVR to always output 16:9 1080p to the HDMI port (4:3 override: OFF), and then just let the DVR upscale everything to 1080p, but the TV just sees all incoming content from that port as 16:9 1080p and doesn't have to adjust its resolution, so there are no delays as I switch between channels or start playing recorded content from various channels.
On another thread I read of someone with a HD DTA that doesn't have a 1080p setting having set it to 1080i, again to avoid the TV delays when hitting channels of different resolutions.
My very humble setup: