Depends.
Most sets offer various display modes. Mitsubishi, for example, offers Standard (squashes 4:3 to fit 16:9...adds weight to everyone

), Expand (crops the top of 4:3 and zooms it in to fill 16:9), Zoom (zooms in all directions and still has a squashed look to it), Stretched (keeps the middle portion of the screen in proper aspect and stretches the sides...looks like a fisheye lens on pans), and Narrow (4:3 with side bars). I usually use Expand as it just tosses the screen bugs and the tickers at the bottom of the screen and shows the broadcast in otherwise proper aspect. And, some DVDs will still have the letterbox effect (if it's greater than a 1.85:1 ratio film).
On a 4:3 TV, all HD programming will look letterboxed as well as almost every single DVD (only older films shot in the Academy format - 1.33:1 or those Pan and Scam versions that Blockbuster offers).