This comes up regularly. Tread on car tires helps evacuate water from under the wheels to avoid aquaplaning; that's almost completely impossible on a bike, so a smooth tire is actually what you want - tire compound however can make a big difference. The kicker is that the stickiest tires (like say Pro4's) aren't always the most durable, and the heavy duty more puncture resistant tires tend to have less grippy compound. I really like running Michelin Krylion carbon's as a good blend of grip and durability - they have been phased out in preference to the new Pro4 endurance tire, but I'd give them a go. As others have noted, take your tire pressures down a bit, and avoid road paint, manhole covers etc that become super slick in the rain.

Edit:68 other people have said the same thing in the time it took me to tap out my post. You get the message.