I'm noticing with Timewarner that some shows are not being broadcasted in True Letterbox. It almost seems as they are re-broadcasting the HD version and 'shrinking' it to fit on the SD screen.
Channels that have the worse ratios:
Ch 7: WGBA (nbc)
CH 8: WPNE (pbs)
It really noticeable when they display the opening / closing credits. Mainly for Network Shows, local content appears to be formatted correctly.
PBS - Nova (Missing credits)
PBS - Frontline (Missing credits)
PBS - In Wisconsin (displays properly)
I have a wide screen, and I don't mind the letter boxing when it's done correctly.

I've noticed this, too.
I've noticed this, too. What is happening is that cable companies are taking the HD broadcast from a channel, and then cropping the left and right sides to convert 16:9 down to 4:3 aspect ratio. This format seems to be the new standard for downconversion when digital HD is converted for carriage on SD cable channels.
If you have a digital-to-analog converter box, you can usually control this function yourself by selecting the screen format you prefer. You just have to disconnect from cable and use an over-the-air antenna along with the converter box. If your TV is widescreen with a digital QAM tuner, you should be able to get the HD version of local broadcast channels on your TV without additional equipment. You then would be seeing the images as they were intended to be shown in 16:9 (no cropping).