A long debate has finally and decisively been settled.

Matt Groening, "The Simpsons" creator and Portland native, tells Smithsonian magazine that he named the famous fictional city after the Oregon city.

The relevant section of the interview:

[Groening:] Springfield was named after Springfield, Oregon. The only reason is that when I was a kid, the TV show “Father Knows Best” took place in the town of Springfield, and I was thrilled because I imagined that it was the town next to Portland, my hometown. When I grew up, I realized it was just a fictitious name. I also figured out that Springfield was one of the most common names for a city in the U.S. In anticipation of the success of the show, I thought, “This will be cool; everyone will think it’s their Springfield.” And they do. You’ve never said it was named after Springfield, Oregon, before, have you? I don’t want to ruin it for people, you know? Whenever people say it’s Springfield, Ohio, or Springfield, Massachusetts, or Springfield, wherever, I always go, “Yup, that’s right."
The Simpsons