A portion of a Game Informer magazine interview with Eiji Aonuma...
GI: Have you been consulted at all for the usage of Link or Sheik or Ganondorf for Smash Bros. Brawl?
Aonuma: I’ve been working with Sakurai for a very long time with this new Smash Bros., because the Wii came out and when discussion for a new Smash Bros. took place nobody could think of anyone other than Sakurai working on it. He was kind of the default, and I was very happy to hear that he would be working on it. Actually, my designers did work on the designs for Sheik and Link and Ganondorf. So they submitted the initial designs, and so it would fit in the Smash Bros. Brawl environment, they’ve had to tweak some of the designs. But Sakurai has brought those altered designs to NCL. We’re working very closely with the team of Smash Bros. Brawl to make sure the characters look their best.
I classify this as a rumor because it's not officially confirmed. Game Informer is pretty reliable, but still take this with a grain of salt.
[UPDATE] Go Nintendo reported the article after I sent in the news story and they seem to agree that it is true. Here it is: http://gonintendo.com/?p=22340