Nintendo's online offerings are free (and somewhat limited) as of now, but that might not always be the case. According to statements from Shigeru Miyamoto to Edge, Nintendo is still considering the nature of its online business.
"Probably the other thing that we are desperate to realise is the core [online] business structure," Miyamoto said. "Do we need to demand customers pay monthly fees to enjoy online activities? Or give an online subscription that is free of charge, but then offer something extra for people that pay, so that they get some extra value? With these core business strategies I think we are less active than we should be."
Miyamoto denied claims that Nintendo isn't "proactive when it comes to its online strategy," noting that the company intends for all consumers to be connected through the Wii at all times (Nintendo even initiated a reward program for those who help others get their Wiis online). He did admit that "Nintendo hasn't been very proactive in developing such online activities as an MMOG."
None of this is any confirmation that Nintendo's going to start charging for some kind of pervasive network, or that we'll see an Animal Crossing MMO at E3. But it is definitely interesting that the company is showing an interest in online services beyond the somewhat barebones setup it has now.