E3 will be taking place next month, from June 5th to the 7th, and Nintendo will giving their main presentation June 5th at 9 AM PDT, 5pm GMT. This will be the presentation that should unveil the final form of the Wii U, as well as most of Nintendo's new game announcements. Nintendo will also have a software showcase on June 6th.
Nintendo is providing us with an official live stream here: Official Site - Nintendo All-Access at E3 2012
You can also watch it live through Nintendo's official Facebook page: https://www.facebook.com/Nintendo/app_385608378148291
Another good option for online streaming coverage is GameTrailers.com, through their GT.TV channel: Online Video Game Trailer TV Show Episodes With Geoff Keighley | GameTrailers.com
This site will cover not only Nintendo presentations, but also Microsoft, Sony, and many of the third party developers as well.
For those who would prefer to use their television, you can watch E3 on SpikeTV, MTV2, and G4.
Lastly, for those who will be unable to watch the E3 presentations live, we here at friendcodes.com will try to keep everyone up to date with the latest info as its revealed.
You can also go to Nintendo Twitter and Youtube pages thougout E3 for news updates.