Not so sure about this. Good programmers can change jobs very frequently. I know many people who have never stayed at the same company more than a couple of years. That's just part of their ambitious, challenge-seeking nature, the will to improve and take things to the next level - it's not really an indictment of the places they stayed before.

I'd be suspicious of a departmnt with a turnover rate around 6 months, sure, but if it was 5 years I'd be equally worried, because unless working for that company was the shit then I'd be concerned with the kind of developers the company had hired in the past and what kind of culture such a bunch of unambitious seat-warmers had developed, and I'd be expected to fit into.