The number of sources which scan for port 80 have been increasing. Please take a look at your web logs to check for any unusual activity.
http://isc.sans.org/port_details.html?port=80
The number of sources scanning a specific port is usually a very sensitive indicator for new self propagating attacks. Given the significant background noise from older worms (Code Red, Nimda, Nachia), it is not easy to spot a new attack.
http://isc.sans.org/diary.html?date=2003-12-07

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