I commute to downtown Chicago, and cannot avoid the steel grated bridges. It's a very unnerving feeling having your rear wheel slip left and right uncontrollably. Countless are the stories about bicyclists falling and hurting themselves during wet weather. To me, crossing a wet steel grated bridge is the true test of the grip qualities of a tire.
I only trust Schwalbe Marathon Supremes on wet bridges. With any other tire (I also have bikes with Vittoria Randonneurs, IRC Metro Duro and Specialized Nimbus Armadillos) I take the sidewalk when the bridge is wet.