You're the second person on FR that has told me that. The article I read didn't mention that but, said it would be visible through telescopes. There will be a number of serious 'scopes' on it though...The Hubble, Chandra and etc. will be viewing/recording the impact. We'll get some very good pictures. The article I read said that images of the incident would be on the internet 15 minutes after impact.
This will probably lead to some confusion. The target is about 2 AU from earth, which means that light will take about 15 minutes to arrive here after impact.
As to whether the dust cloud of the impact will be visible: the comet is magnitude 9.7 right now. The limit of naked eye visibility is mag 6. If the dust cloud is 6 or brighter, it should be visible. But this brings up the problem of light pollution. Many people rarely see stars dimmer than mag 4 or even 3 due to light pollution. If there were no light pollution then many could see the event. With light pollution, few would see it. I expect sightings will be rare in the extreme.
It will be extremely easy to find, though. Giving directions should be a snap.
Guilty! :-)
But I swear I heard that on my local radio station!
It will be a new moon, though, which I believe would have been included in the mission parameters deliberately.