YouTube seems to have switched their standard playback format to widescreen. All 4:3 videos have the horizontal black bars on the sides. Checking YouTube today resulted in some showing the widescreen playback format and others with the old-style 4:3 format so I'm not sure if they are just testing or if that's due to transitioning to the new format.
An older YouTube Video uploaded in 4:3 format shows the black bars on the sides
A newer, widescreen video now shown in widescreen on YouTube by default
This along with YouTube's move towards full-length films (so far just from MGM) all indicate a move to compete more in the Hulu space. There are plenty of other video sharing sites with widescreen as the standard format including Vimeo, Blip.tv and others. YouTube moving this direction is big since it is the best known and probably most popular of the online video sites.