Nintendo plans three playable demos for Pokémon Mystery Dungeon: Explorers of Sky, the first of which arrives this Friday 13th November.
A further sampler will appear each following Friday (20th, 27th).
These are DS demos, and they will be available from the Wii Nintendo Channel, which can be downloaded from the Wii Shop Channel.
From there, click "Find Titles For You" and then "DS Download Service". Select the Explorers of Sky demo. Now turn on your DS and select "DS Download Play". Accept the download when the DS connects to Wii to complete the quest.
Pokémon Mystery Dungeon: Explorers of Sky won't be out here until 20th November.