On Tue, 09 Nov 2010 17:20:02 -0500, HarryK wrote:

The North American Aerospace Defense Command, or NORAD, issued a
statement jointly with the U.S. Northern Command, or NORTHCOM, saying
that the contrail was not the result of a foreign military launching a
missile. It provided no further details.

Of course that implies that they know what it was - probably some
classified test.