Bryn makes a good point. If possible it's an idea to let 'er rain for a half hour or so before heading out. That way the oils lifting from the road have a chance to wash away. As you yourself pointed out Scott, avoid manhole covers, and painted surfaces... come to a stop BEFORE the painted line across the road at an intersection for example. Use caution on railway tracks, bridge grates, wood (as in old bridges) and here's a good one... WET LEAVES!!
It's surprising how much traction there actually is on a wet road. The easiest way to test traction is to feel for it with your rear brake. Assuming you know how much deceleration you get on dry pavement before the rear tire breaks loose, you have a gauge of what's available if you repeat the test when the road is wet. If you do this at moderate speeds on a flat, straight road it won't become a thrill ride.
Other thoughts... still use the front brake but use it gingerly. The obvious... run with good tire with decent tread. Dress properly for the occasion... being cold and wet slows reactions and causes the rider to stiffen up. Maximize the ability to see but turning your head from one side to the other to help blow the rain away from visors or goggles.
I also try NOT to ride in depressions made on high traffic roads by the weight of vehicles rolling along. Yeah... getting in there can cause hydroplaning which is no fun at all!!
