Late to the thread - but I've done a fair bit of barefoot running.
Two 'Great North Runs' done barefoot. That's a pretty harsh environment, 10's of thousands of other runners, bitumen covered in plastic bottletops etc.
I wouldn't use the vibram things. When running barefoot, you avoid stone bruises and turned ankles by feeling something before your weight goes on the foot. It is a reflexive thing to do a 'stumble' and avoid a bruised foot, turned ankle or broken toe. Stick some rubber on the outside of your feet and you lose that sensitivity.
One of my ankles is damaged from a nasty (not)rock-climbing fall - I'm missing several important ligaments on the outside of the ankle and the joint is misshapen. This hasn't prevented me from running distances of up to 14miles on and off-road.
I took it steady getting back into running. Allowed a similar buildup as those recommended to avoid shin splints and found my feet toughened up quickly. At the peak of my running barefoot I could tread on brambles without the thorns penetrating the calluses. I've got home and picked broken glass out of my feet - no blood, the glass hadn't penetrated and I hadn't felt it.
The downsides:
Running downhill on hard surfaces is much harder on your knees. With running shoes on, you can let a lot of the impact be taken by the cushioning in the shoes. Doesn't exist when barefoot, so your knees have to do the job.
Wet and muddy ground. Wet softens the skin, slipping in mud can result in cuts when there are sharp stones, sticks or broken glass in the mud.
Scree. You'll break toenails in a bloody way, but will be much more light of foot and faster than wearing boots.
If running with other people, you feel short. Being barefoot loses over an inch in height.
You get called a hobbit.
The upside:
It's cheap (have you seen the cost of running shoes?)
Less tendency for shin splints
People think you are hardcore (I got a shout out on the big screen at the GNR).
No washing mud off shoes or griping from the missus about stinking running shoes in the house.