Just like in 1966 most new content was in color but some was still in black and white, most new content now is in 16x9 but some is in 4x3.
Universal HD has several programs in fabulous HD that was not originally filmed in the 16x9 format. The reruns of Northern Exposure are remastered from the original film (which has higher resolution that even HD TVs). But the original film is 4x3.
So, even though Universal HD is 100% HD, there is no up-coverted material on this channel, all the programs are remastered from original film stock and is true HD, some programs are 4x3.
At the same time, HBO, Starz and Showtime HD channels sometimes show movies in the 2.35:1 ratio. This means that these movies are letterboxed on a 16x9 screen too. So I guess these wouldn't count as 16x9 either.
These channels also run original programming that is not shot in HD but still looks good (a hell of a lot better than the SD versions) like Real Sports, Bulls---!, Bill Marr, and some sporting programs in 4x3.
A&E is the king of upconverting programming. Basically you get the SD programming on the HD channel where you essentially have no (perceived) loss from the original SD master but the program is clean. CSI Miami is the only real HD program I have seen. But admittedly, I really don't watch that channel much since they lost the L&O franchise.
The Voom channels generally have everything filling the screen with some exceptions on the movie channels. They show their movies in the original aspect ratio. So some movies are pillar boxed and some are letterboxed, but for the most part 16x9. HOWEVER, some programs especially in the family and cartoon channels are made 16x9 by hacking off the top and bottom of a zoomed in 35mm film (IIRC 1.37:1) which is just a little wider than the standard 4x3 (1.33:1) screen. This is how "Flipper" and "Thunderbirds" are made 16x9. By REMOVING portions of the top or bottom or both. Essentially tilt and scan instead of pan and scan.
So I guess the short answer to your question is MOST channels have at least SOME content that will have some form of black bars on your screen. However MOST channels have MOST of their programming filling up your 16x9 screen with picture and no black bars.
See ya
Tony