Source: Me.
It has not been publically announced, but it is in the works behind closed doors. Do not be surprised if you hear more about it closer to the release of Super Smash Bros. Brawl.
Personally, I feel they're going about it the wrong way, since the DS has the larger user base. Put Smash Bros. on there first, get people hooked, promise extra crap if you connect with the Wii version and people will buy a Wii/Brawl. I am not sure if there will be the incentive if they do it vice versa.
Oh well, I don't run Nintendo.