I don't intend to look into the online capabilities much, but I'll tell you what I have heard. They suck.
Right now, you will have to trade special codes with your friends for each game that you will want to play online. From my understanding, there will be no playing random people. You have to trade codes, that means each person has to enter the code on their machine before you can play. Kinda defeats the purpose of online play. They claim that it was to protect people new to online games (like the elderly and little kids) from getting flamed and insulted by the normal idiots who hang out online. But people are complaining, so hopefully things will change.
Oh, and Wii sports doesn't go online. I personally don't mind, I'm not much of an online gamer. But if you are, and that's why you are getting a Wii, then maybe it's not for you.