Remember that it's going to take longer to stop, drivers will be even worse at seeing you, take it easy on corners, watch out for tracks and painted lines/manhole covers, etc.

And once you're used to that, biking in the rain can be really fun--splashing through puddles like a little kid, feeling like you're a badass for being out at all--good times.

If you really want info, post in the Pacific Northwest forum asking for rain-riding tips. We wouldn't get much riding in up here if we avoided biking in the rain...

EDIT: Oh yeah, lights and fenders are a really good idea.