Yes! Nintendo confirmed this at E3. Because of the Wii's unique control system, it may be possible to play DS games on the Wii. It is also possible that the Wii and the Nintendo DS will connect like the GBA and the GameCube did, though I am not sure how this would work. One thing that Nintendo has confirmed is that you will be able to download demos with the Wii, which you can then transfer wirelessly to the DS. Many more forms of connection will probably be available, but we will just have to wait.

Fortunately, no. Most of the games out for the DS use only one cartridge for multiplayer games, using a feature of the DS called single card download Play. This feature allows one DS to transmit all of the data necessary to play a multiplayer game to another DS, which can then play against it. Some games even allow you to download demos onto another system. The demos usually don't have much on them, but they do allow other people to try out a game before they actually buy it. Who knows?Perhaps Nintendo will transmit demos via online play so that gamers across the world can test out games whenever they choose to.

Yes, you can attach them, but they don't actually do anything. Kind of a bummer, too, since using that on-screen keyboard of theirs is kind of a pain -- we often aim off our letter when pressing the A button to type.