Good topic.
This is one of my biggest disappointments with this generation. The DS has enough power and memory that it could be used for something neat... but most dev teams seem like they don't care enough to try.
GameCube used the GBA-Cube connection sparingly because of special cables required and a much lower install base... but still had some tremendously enjoyable titles take advantage of the option. Zelda (Windwaker and Four Swords), FF:CC, Pac-Man Vs, Animal Crossing, and a couple others made good use of the connectivity. Many games provided token use, but that's still better than nothing.
It's shameful that Wii-DS connectivity hasn't seen any serious follow-through on Wii, especially since it should be easier and more flexible for both dev teams and gamers. Why isn't Pac-Man VS available on WiiWare*? Why couldn't Zelda: Four Swords be part of the New Play Control series? Why didn't Metroid Prime Trilogy allow users to download the original Metroid game as a demo for your DS?
*WiiWare in particular seems like a great place for this sort of development. Development and distribution costs are cheap, and you can sell "glorified tech demos" much easier when people are only paying a couple dollars... So far, the only WiiWare titles I know that use it are Mobashi arcade and Pokemon. Are there any others?
Oh, and don't get too excited about the Wii Final Fantasy game that allows you to play with people on DS systems. The Wii version of the game is an exact translation of the DS version, and not a centralized "host" that allows gamers to use their DS for individual controls/screens. The two systems can't even use local wireless, and must connect over the internet instead to start a game. A Wii-DS combo here behaves exactly the same way that two DS systems would.