I watch SD cable on my widescreen TV and I've noticed some programs have black bars on the top and bottom of the screen. I am familiar with aspect ratios and it would seem that I am viewing programs with a 2:35 aspect ratio. Why would this be? If these were simulcast HD programs, wouldn't they be in a 1:78 (16x9) aspect ratio? Thanks in advance.

It's called "windowboxed" (or postage stamped). 1.78:1 programs on an SD channel will appear in this format.

Use the zoom feature on your TV or STB for those channels.