I come from Pittsburgh so I am particularly attuned to mentions of cyanide in the press since we've recently had one prominent researcher murder his physician wife using the substance as well as another researcher use it to commit suicide.
News that The White House received a cyanide-laden piece of mail should not, however, lead to widespread panic as cyanide, like ricin (which has also been concealed in recent mailings to several government officials), is better thought of as a poisoner's tool rather than a mass casualty inducing substance (though it was used in gas form in Germany's WWII concentration camps). I often describe such substances as weapons of mass hysteria rather than mass destruction. In the context of mail, the intended mode of exposure would be via the skin which is less effective than via inhalation or ingestion. It appears that this attack originated from someone known to government officials.
Cyanide is a simple substance that is relatively easy to obtain and is a well-known component of house fire smoke as it is formed from the burning of carpet. Cyanide's deadly characteristics come from its ability to block the ability of cell's to utilize oxygen to make energy. This property essentially halts all cellular activity and ultimately leads to death. There are antidotes to this poison, but they must be administered quite rapidly to have any effect.
An interesting aspect of the nefarious use of cyanide is the past effort (which thankfully never came to fruition) by Al Qaeda to develop a device to disperse the cyanide gas in a manner capable of causing mass casualty in the New York City Subway. Finding this device, named the "mubtakkar", and uncovering the plot to use it became a major priority for the Bush Administration.
Anytime a biologically active substance is used in the initiation of force it is a major cause for concern and should reinforce in the minds of all how easy it is use these substances as weapons. While 14 years have passed since the anthrax attacks, cyanide and ricin biocrimes have been occurring with increased frequency, highlighting the low barrier to their acquisition. Preparing for such events by heightening awareness, improving diagnostic capability, and developing effective countermeasures must remain a priority.