Over at catallaxy, Jason Soon brings to our attention an article in this month's Quadrant (link at catallaxy) on this subject.
Go have a look.
I wondered about why there were so many foreign doctors in Australia for no other reason than that the countries they came from would need their services more than Australia as the doctor:patient ratio in many of these countries would be incredible. I wondered why they didn't stay in their country of origin where they and their skills were needed. I was set straight by a commenter at Tim Blair's (MareeS I think). The reason these people come to wealthy countries to practise their trade is because the pay is much better and they can send back money to pay the family debt they have accrued from their study and help their families, particularly in countries where there is no welfare safety net and incomes are very low.