In Unity with Wii Development

Short Story: Unity can be used to develop games for Wii. To do so, one will need to: (1) be a licensed Nintendo developer and/or have a Wii Dev Kit, (2) have special Wii Dev license from Unity.

Long Story:
On The Nintendo Side
The process of becoming a licensed developer is the same as it has been for the major consoles for many years: (1) if one can demonstrate the ability of one's company to properly handle the license and bring a product to market, one is eligible for a developer license; (2) the console developer remains fairly involved in the development process (they require that the requisite bug reports be submitted to them, etc.); and (3) the cost of obtaining a license is not disclosed, and one finds out only if/when one qualifies.

An Nintendo statement to this effect can be found here:
http://www.warioworld.com/licensing/becomelicensee/

Here is some of the key text from that page:

"Please send a corporate portfolio consisting of the following information:

1. A detailed description of your company...
2. A detailed introduction to your key personnel...
3. A marketing plan for your proposed products...
4. Any market study information on consumer demand for your proposed
product...
5. A written description of your proposed software product;
6. A complete summary and at least three samples of software you have
previously published...

"Nintendo will then undertake to make a determination if your qualifications would support your selection as an authorized licensee..."

On The Unity Side
If one is a licensed Wii developer, one can approach Unity about obtaining the necessary code (controller class, etc.) for publishing directly to the dev kit. Like the Nintendo license, the cost is not publically disclosed, and one finds out only if/when one qualifies.

The Unity "Nintendo Wii port of the runtime has now been in development for well over a year."Unity has been working with certain developers doing beta testing of their Wii API, "several months". The announcement of official "Wii Middleware" status was made just yesterday. In his announcment, the president of Unity directed interested developers to the official Nintendo "Software Development Support Group" web site at http://warioworld.com/

Website announcement:
http://unity3d.com/company/news/wii-press
http://unity3d.com/unity/features/wii-publishing

Community forum announcement:
http://forum.unity3d.com/viewtopic.php?t=11835