Here's an interview with Ian McShane regarding it being cancelled
http://www.contactmusic.com/new/xmlfeed.nsf/mn dwebpages/mcshane%20fuming%20over%20deadwood%2 0cancellation_1000801
I also found thi statement from HBO
Despite recent articles on the future of our HBO
Original series "DEADWOOD", there has been no
decision for the future of the series, and
conversations regarding a fourth season are ongoing.
Since "DEADWOOD" creator David Milch is working on
another HBO Original series, we've granted our
beloved Deadwood cast the latitude to pursue other
projects, for the time being.
We appreciate your interest and hope that you will
continue to find HBO to be a source of quality
entertainment.
Sincerely, HBO Consumer Affairs Department