Two respectable US sites are listing a Gorillaz US album release for March. Firstly, billboar.biz (the Billboard industry site) lists a Gorillaz album 'title TBA' coming out on their US label Virgin, on 6 March (link to the listing is here, thanks to Alex for sending it in). Secondly, US record releases site musictap.net lists a Gorillaz B-Sides album as coming out on March 20 (link to the listing is here, thanks to 87th for sending it in).
Further information will be posted here as it becomes available. At this time no release has been officially confirmed.