Nintendo used E3 2011 to debut Wii U. It will use E3 2012 to show the system's final form.
During an earnings briefing today, CEO Satoru Iwata said that Nintendo hopes to show the "final format" of the Wii successor at next year's E3. The system is planned for release in the next fiscal year, which means post April (although if the final form will be shown at E3, we can probably take this to mean post June).
Iwata mentioned Wii U only briefly at the presentation, also saying, "As we learned a bitter lesson with the launch of the Nintendo 3DS, we are trying to take every possible measure so that the Wii U will have a successful launch."
You can read Nintendo's full earnings presentation here.