Taken from Gorillaz info:

Gorillaz and D12 have joined forces to record a track in reaction to the September 11th attacks on the United States. Entitled 911 it was recorded in London in mid-September after D12 were stranded in London, unable to return to the US. The track deals with the extreme violence of the attacks.
Also featuring Terry Hall on vocals, 911 will be available as a FREE download from the official websites, www.gorillaz.com and www.d12online.com from December 7th.
Gorillaz' bass player Murdoc said of the recording:
"Working with D12 was a reassuring experience. I like their work ethic - turn up en masse to the studio with a whole bunch of good looking girls, bags of grass, champagne and sweets and just lay it down baby. Beats the average day in the Gorillaz studio, watching 2D walk into the sound proof glass sliding doors every time they're shut, listening to Noodle's Fisher Price DJ tables whilst having to constantly smell Russel's deadly bum vapours. D12 are CRACK OR IS IT WHACK! Oh f*** it they're Wicker-man!!"
Swifty of D12 added of working with Gorillaz: "It was cool. They showed much love. We chose some beats, we went on a vibe, a nice vibe. We enjoyed working with them, they have a nice flava."
Terry Hall also paid tribute to his collaborators: "I met Damon a few years ago when he was first staring out with Gorillaz. We always said we'd like to work together and this just felt like the right time. When I first heard what D12 did I was amazed, such unedited emotion. The track is all about opening your heart and your mind."