"Formal Qualifications This is more a of non-indicator than a counter-indicator. The key point to outline here is that formal qualifications don’t mean squat when you’re trying to recognise a good programmer."
Qualifications is an effective way to limit at least some of the people. Most 4 year university degrees require at least a basic level of intelligence and commitment. It is true that there are a lot of professional Comp. Science bullshitters out there with degrees (I know quite a few) but on the average you will be luckier with a university guy. This does not just mean that you should look for Computer Science – applied mathematicians are also a good bet (based on other qualifications). (Of course there are also excellent programmers without degrees – but it is extremely difficult to find).
Another thing – something as simple as a typing test may help you. There are people out there that claim to be programmers (esp. freshly out of college) but do not really program (further than the task at hand). People who programs a lot for a long time will generally type faster than people who do not really program.
Also I think that to hire someone is fairly expensive (at least $50,000 a year in the states). If you make a bad decision it is a lot of money down the drain and one bad guy can ruin the atmosphere of a whole company. Why not hire a programmer (that you know to be competent) to help you with the interview? I am sure that a $1000 expense is less than the risk of wasting $50,000.