"I DON'T AGREE WITH THE ETC GROUP ON NANOTECHNOLOGY, but I think they're right to call for caution before undertaking iron-seeding of the ocean as a means of stimulating plankton growth and carbon sequestration, something I mentioned a while back."
http://www.instapundit.com/

I don't see how they are right to call for caution:
"Planktos, a self-styled 'eco-restoration' firm that also doubles as a nuclear fusion company(3), intends to dump tens of tonnes of tiny iron particles over 10,000 square kilometres of ocean around the Galapagos Islands at the end of May 2007."
http://www.etcgroup.org/en/materials/publications.html?pub_id=617

Tens of tonnes of tiny iron particles hardly seems to be very significant. First, iron is very common element- you could some dirt have 5% by weight iron content- so it would about a significant as dumping a couple hundred tons of dirt into the ocean.
And in dredging this done all the time- the only difference is he might dumping this in a part of the ocean which not like coastal areas which have plenty of this type of material. I don't think any result will be easy to measure.
Now, it's a bit loopy, but the only thing I see as dangerous is the future false claims of the potential resulting and then potential of pseudo science and "mad rush" that might spring from this little adventure.