There's a press release claiming that The Last Airbender: Legend of Korra and other new series will premiere in 2012 on Nickelodeon. The article can be found on the Belgium MTV Network website, written in Dutch. Posted on December 13, 2011, this is so far the lastest article to mention when Legend of Korra will premiere.
(last paragraph) Done by Google Translator
...In 2012 there are already lots of new series in the pipeline. So in the spring brings Nickelodeon series "Life with Boys", a live-action series from the creators of Hannah Montana. In the fall is "Legends of Korra", a spinoff of the hit series Avatar, and "Teenage Mutant Ninja Turtles", a remake of the same name and original animation series from 1988, on the program.
The website's affiliation with Viacom and Nickelodeon appears to be legit, so this can labeled as reliable information. On the Belgium MTV Network's Twitter page, it states to be "part of Viacom Inc. a worldwide expert in kids & youth entertainment through its brands MTV, Nickelodeon and TMF."
And on a page of its own websites it claims to be "a unit of Viacom." 
One of most recent press releases on the website says, "For the Belgian market, Viacom International Media Networks signed a partnership with SBS Belgium, and from 1 January 2012 will be responsible for the commercialization of channels TMF, MTV, Nickelodeon and Nick Jr". (December 20, 2011)
Please stay civil. You should not harass other users or their views.