In an article to give hope to all of us that looked forward to Ghost for so long only to have it cancelled, Blizzard talks about the game and how it "may" come back someday.

“Our most recent one was Starcraft Ghost. With that game we were very stubborn. I still believe in that game and the characters but we were not able to execute at the level we wanted to…Rather than work on that we had to focus on our other games. We're hoping one day to return to it.”

Check it all out here: Blizzard Still Has “Hope” for Starcraft Ghost.