Keith Olbermann's Foot on the David Letterman show.
It seems they may cause stress fractures. Keith Olbermann appeared on the David Letterman shoe wearing a “boot” due to a stress fracture in the VFFs. There is an ongoing discussion on a podiatry forum I found online. In regards to stress fractures and VFFs, the podiatrists were not the least bit surprised that people were reporting injuries from running in the VFFs. Though to be fair on this one, they sounded as bad as the “church of the barefoot runner” folk do when talking about it. They are hostile, non-helpful to people who might have inquiries about them – and just plain mean at times. They actually search the internet (like my google search earlier) looking for people reporting foot breaks in VFFs, and then they mock them. So I take them with a grain of salt, just as I do the “VFFs/barefoot running will cure all your problems!” Plus, add in the fact that if VFFs really do cure alot of foot problems – it will seriously cut in to a Podiatrists’ bottom-line.
Personally, I know my knees hurt less in the VFFs than in traditional running shoes. My spot for my hernia surgery hurts more today than it has after any past run in the VFFs. Those that support VFFs say that stress fractures are due to rushing it – and doing To Much To Soon (or TMTS as they say it..)
As a quick aside – I don’t get why, on the barefoot/minimalist forums any injury in VFFs or Barefoot are TMTS, but any injury in running shoes are the shoes fault.. I think there is a jump in logic I am missing there.
It puts me in a quandary for my marathon training. I have gotten up to 7 miles in my VFFs before my calf injury. I really don’t want a stress fracture in my foot – but on the same note, I really enjoy not having knee pain. As far as research, people seem to be amazingly for them, or horribly against them – with no middle ground. You would think with our advances in technology and science, we would do studies on things, rather than both sides sticking to anecdotal evidence.
I run, because I am a runner. Having realized that I have always "been a runner" - even when I went years with out running, I am learning how it has integrated itself in all aspects of my life. When I started running in highschool, my perspective on life changed. Funny that I didn't realize how much it had affected it until just recently.
Once and Future Runner 
as an update – I have chosen to only run in the Vibram Five fingers. I am concentrating on a midfoot strike, as opposed to forefoot – this way the force of landing is evenly distributed across the bottom. I will be running a 15k on sunday in my VFFs!