Nintendo plans to offer a free online model with the WiiU as stated by Nintendo president Satoru. The free online model is expected to increase the user utilization more-so than a subscription model would allow.
On the competitive side of the coin, Sony has seen great benefits from offering a free online model through their Playstation Network service. Eventually Sony offered users the option of a subscription/membership via Playstation Plus allowing users to experience additional updates such as free game trials, exclusive beta access, free AAA titles, etc.
How long Nintendo plans to offer a completely free online model is unknown. Nintendo also has no plans of integrating Facebook or Twitter into its Miiverse, a move that points at Nintendo possibly looking into building their own social network as well.
This holiday we’ll see where Nintendo takes us with the WiiU.