Hm. I had this problem, or close enough, yesterday. I had tried adjusting it before, but apparently I had slightly dished the wheel. What I did was remove the tire, put the wheel back into place, center the brakes, and then loosened all the spokes so they were incredibly loose. Then I went around and tightened them a bit, and just kept going around until it was true, using the brakes as a guide. Took me about an hour or so, but it's pretty darn straight.