Online will work out fine for Wii. It's not going to be the main selling point anyway. Look at DS, people are flocking to online with it, but it's because the DS offers everything plus free online games, not because it offers a huge fancy online experience like Xbox 360.
Online will come and it will be good enough, but I'm sure Xbox 360 is going to offer a better experience - the problem is you have to pay for it. Overall I don't think this will hurt Nintendo too much anyway, because online has relatively little to do with what makes Wii an exciting system. Probably a few hardcore gamers will ignore Wii or buy Wii and another console with better online, but I don't see it hurting Nintendo much.