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.