Native functions to ease the creation of contests would be nice, I support. It'll take a while to implement though. In the mean time, you can organize your own competitions using the Forums to advertise it and gain entrants, and store entries in a gallery.
I know thre is Collab Camp which goes along the lines of this and displays the winning projects, but I personally have not been on Scratch long enough to enter it, and ST may be waiting for the full release of 2.0 to start the next one.
Anyway, I agree with Gravitation, although the success of the contest would depend on a number of things.