Swisscom (Switzerland) says february 2012, other countries perhaps earlier

Source:

http://nokiagadgets.com/2011/12/10/rumor-some-new-info-about-lumia-900/

http://wmpoweruser.com/tag/nokia-900/

http://mynokiablog.com/2011/12/14/nokia-lumia-900-coming-by-the-end-of-february-2012-to-swisscom/