Hopefully I'll summerize these okay. Here it goes.
Terrence Sejnowski says the Internet will eventually become Skynet (as in self aware). He's go some stats which suggest it's probable.
Jaron Lanier has an interesting one that humans, inside a virtual reality simulation have no problem learning to adapt to using a non−human body. Like a body with 5 arms instead of 2 or 6 legs. So, the question is, when science actually allows us to do these things either in VR or even in the real world through genetic engineering or robotics what will the world be like?
Irene Pepperberg's Idea is that basically it will be proven there is no real difference between humans and other animals. Currently most people believe there are some specific differences between humans and other animals. Like the idea that we can think or we can comtemplate our own existance or we can use tools. Whatever it is we use this idea to separate ourselves from animals. But, what if it can be proven there is no specific difference except one of ammount. In other words instead of an animal not having whatever specific traits make us special, instead they have those traits just at a lesser level.
Douglas Rushkoff has one which I might sum up as because of tech, more and more people bartering instead of using money. I'll fix your computer if you take a look at my tonciles. The dangerous part I guess is how the government and banks become less relevent if everyone is going around them.
Juan Enriquez wonders if Tech will destroy the USA. His argument seems to be that basically all countries die eventually and so is the USA still a kid, in middle age or in old age and is tech extending its life or shortening it. He's got some interesting stats to argue it's not inconceivable.
Geoffery Miller has an interesting one. I don't think I knew (or I forgot) about Fermi's Paradox. It basically says, if the universe is as old as we think it is, and if life happens on other planets in the same kind of time frame has it happen on Earth then there should argablely be lots of aliens. If so, where the hell are they? Either the odds of life are a lot smaller then we believe OR they all some how didn't make it (blew themselves up or whatever). Read his article for what he thinks might be up.
Judith Rich Harris has a scary one which is that there is mounting evidence that parents have zero influence over their kids and how they will turn out.
Freeman Dyson has one I mentioned before which is that advancing tech will make manipulating genes as simple as manipulating computer code is today. The danger being that tomorrows gene hackers will make real viruses just like today's computer hackers make computer viruses and end up killing off the world.
Jeremy Bernstein has one which I'd like more info on. Basically he says we don't understand Plutonium and that all the bombs that use it could explode at anytime without warning.
Clay Shirky and several other people suggested we will prove we have no free will and that will change what we can hold people accountable for as well as other ramifications.
James O'Donnell talks about how goverment will disappear as people get less and less dependent of the things government has traditionally provided. It's not clear how this is a dangerous idea.
Matt Ridley has a related one which is that it's proven government is the problem, not the solution. I tend to agree. It's also not clear how this is a dangerous idea.
J. Craig Venter and several others say will are on the road to proving genes determine your behavior, something most people would rather not know and there are quite a few dangers in knowing but that we will know relatively soon and so we'd better get ready to deal with this new knowledge.
I hope I didn't do a too poor a job in summerizing some of those. They certainly are thought provoking.