I can't see it really being a problem.

The magnets in motors aren't that strong. Having near a big speaker would be way worse. I don't even think the motor in a treadmill needs to have any significant power (thus doesn't need a strong magnet).

I didn't read much of the DSL thread (I'm assuming it's the same one) cuz they're basically all speculating. Anyway, it shouldn't be radiating significant interference because there are FCC guidelines. If running the treadmill produces a voltage lag it shouldn't matter because of the AC/DC transformer that powers your computer. Putting the PC on a surge protector is always a good idea anyway.