It's more a matter of competition. I've worked for companies that picked Portland specifically because it had plenty of developers, but not as many employers, so the supply/demand dynamics favored a lower salary vs. other areas.
I suspect that a direct calculation will show that Portland is more expensive, but the right type of person can live in Portland very happily on much less money than in Houston. Portland really promotes bicycling and light rail, while in Houston urban sprawl is the norm, and a car is virtually a requirement. Between the depressed Oregon economy and the somewhat bohemian culture that is modern Portland, there is probably a self-reinforcing lull in salaries.
