The phrase "canary in a coal mine" refers to the practice of 19th century coal miners who would take canaries into the coal mine with them as a means of detecting dangerous gases. Since the canary is much smaller than a man it is more sensitive to gases such as methane. If the bird died, or stopped singing (as Kurt Vonnegut once put it), that was a sign you had better get out of the mine.
It is a tragic irony that the phrase has significance in relation to the recent Sago mine accident in which 12 men lost their lives. The mine had been cited with 270 safety violations in the last two years.
The relevance is that one of the Bush administration's first actions in office was to get rid of a canary who might warn of potential danger in a coal mine.