Nintendo still have a lot of moves they could make to continue selling Wii hardware, including a price drop, new bundles, new colours, etc. There isn't much reason to release a new system this year when their current one is still selling well enough, especially given the launch of 3DS. I wouldn't be surprised if there were some mention of a new system before the end of the year, but at least for the next several months, everything will be about the 3DS.
The GCN and GBA is the only time Nintendo have released two new systems in the same year. In that instance, they really needed to put out new hardware: the PS2 release in late 2000 basically made the N64 irrelevant, and I don't think the Game Boy Color was selling very well at that time either (by Nintendo handheld standards). Even most of their Game Boy and DS hardware revisions were not released in the same year as a new home system, with the only exception being the DS Lite launching in the same year as the Wii.