If you head on over to the official Super Smash Bros. Brawl website you'll discover that the game will include a "Stage Builder" (level editor) which will allow you to create your own stages.
After you have a stage complete you can send a stage to a friend, place it on an SD card for safe keeping, and/or submit your stage to Nintendo.
Each day Nintendo will choose one stage from those submitted and will send out that stage to all Wiis around the world. Each day you'll have fresh content, however, the stage will disappear within one day, making it a "once-in-a-lifetime encounter."
This feature adds a limitless replay value to the game. Nintendo may have just shot themselves in the foot...with a game like this, you won't need to buy any other games.
SSBB is due out in the US on Feb. 10, 2008.