I got the same thing from Norton today, both from running Bitcoin QT and Litecoin QT.
Network traffic from checkip.dyndns.org matches the signature of a known attack.
I've etablished from this thread that there is no threat, but why is this occuring? I have been running Norton alongside QT for over a month and this only occured for the first time today.
same thing just happened to me when I loaded up the litecoin qt, and I've been using the qt + Norton for a few months.
Norton flagged checkip.dyndns.org as high risk worm.