Great points. I have personal connections in some poor rural areas of Kenya already (and already in that place, people are always asking, "How can I join?", both of me and of the current workers). My colleagues also have connections in a number of other developing countries where we could plant "seeds". How to grow the "crowd" from those seeds is an interesting problem, but not insurmountable, what with ever-increasing mobile phone/3G penetration and a mobile interface to our platform (lots of people in Kenya, for example, have a low end mobile phone or internet-capable "feature phone" while still living in mud huts), a franchise model using netbooks and small solar stations, etc.

As for quality control, you're absolutely right. There are a lot of ways to approach that (none of which Mturk implements). There's some well-established precedent for methods that work, so we feel confident we can generate high quality outputs.