Even though most people have more DVDs than Blu-rays and there are more standard-definition (SD) channels than high-definition (HD) on all cable/satellite providers, it's highly unlikely most people send their televisions standard-definition signals.
Nearly all current DVD players, and all Blu-ray players, convert the SD from a DVD to HD. Nearly all cable/satellite boxes upconvert the SD channels to HD. Since this upconversion happens before the signal is sent to the TV, most properly hooked-up HDTVs today only ever receive HD signals.
The only readily available SD source is Nintendo's Wii.
There is a practical thought behind this esoteric question. If no SD is ever sent to a television, is the standard-def processing of a TV, and the associated SD inputs, irrelevant?
What do you think? Do you still have, and regularly use, any SD sources?