Jan Heine at Bicycle Quarterly had a bunch of posts on his blog about the research they have done. They insist that wider is faster within certain ranges. It also goes along with their theory that the pressure has to be appropriate to allow 15% compression of the tire to absorb shock rather than bounce off. It makes sense, I don't dwell on speed but I can run with a 16-18mph paceline as easily with my Sam Hillborne on 32's as my old TREK roadie on 25's. I know my Hunq is faster on 50mm Big Ben tires than it was with 40 mm Dureme, but that is probably the result of a better tred design.
Marc