A good portion of the cast for The Last Airbender: Legend of Korra was revealed not too long ago. Many of the names are attached to characters whose names we have never heard before. Here's the list:

Janet Varney ... Korra: the main protagonist of the series.

J.K. Simmons ... Tenzin: the son of Aang and Katara. He is an airbender like his father.

Dante Basco ...  (rumored; reprising the role of Zuko possibly?)

David Faustino ...  Mako (probably the son of  Mai and  Zuko.)

Seychelle Gabriel ...  Asami

Kiernan Shipka ...  Jinora

Darcy Rose Byrnes ... Ikki

Maria Bamford ... Pema

P.J. Byrne ...    Bolin

Daniel Dae Kim ... Hiroshi Sato

Lance Henriksen ... Lieutenant  

On the downside, this is the second article that has used the long title The Last Airbender:Legend of Korra - an unwelcome possible change of title considering its length and its association with a movie many fans and critics did not enjoy (myself included). However, it should be considered that both articles were from the same blog, written by the same person - Christopher Farley. The Last Airbender Legend of Korra is slated for release in mid 2012. You can read the interview with the creators for more on the show and its progress here. http://blogs.wsj.com/speakeasy/2011/03/08/the-last-airbender-legend...  

(The interview was posted about a month ago. No other news has been revealed since then.)