Mario Tennis and Tetris have some on-line play, but they're frankly not that great. Nintendo put the tools on the table for on-line play, but nobody is using them... Aside from swap note and the brief messages over your Mii in the friend list, there's nothing going on here for on-line play.
Of course this will change when Animal Crossing hits next year, but I was disappointed to learn that the in-game "chat" is the same as on the DS "Wild World" version... typing, not actual voice chat. Super lame.