This apparently is old news, announced this past August: BBC has renewed (recommissioned) Luther for a third season. We learned of it when star Idris Elba mentioned on BBC Radio this week that he had been "definitely given the green light for a third season", which he expects will air in late 2012 or early 2013.
"Ultimately I'd like to see Luther on the silver screen," he added. "I think it's got the scope to do something like that."
The second season of Luther concluded earlier this week here in the US on BBC America. The season DVD comes out next Tuesday.
(Source: BBC Radio.)