I wonder if this has something to do with the shuttle launch?
"Space Command would not comment on the reason for the security increase."
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I think if it were the Shuttle Launch, they would probably say so.
Of course they are only at a "slightly" heightened alert, but better safe than sorry, so I hope they are watching what they need to be watching.
Note: The following new brief is a quote:
http://www.nasa.gov/externalflash/sts-121_front/index.html
Discovery Launch Set for Sunday
NASA plans to launch Space Shuttle Discovery on Sunday at 3:26 p.m. EDT. Saturday's launch attempt was scrubbed due to weather conditions at the Kennedy Space Center.
During their 12-day mission, Commander Steve Lindsey and the Discovery crew will continue evaluating new safety procedures during a visit to the International Space Station. At least two spacewalks are planned.