Same as Salsabike says.

I always ride a planned ride in the rain, I ride through rain in the winter, and often in the summer (including a lot of rain last weekend - the Gulf Islands are unpredictable!!). My bike club (in Vancouver) only cancels a ride if it's below freezing level about 3 hours before the beginning of the ride (because the frozen pavement is not very good with road bikes).

Good maintenance and care of your wet bike is important, although you shouldn't loose sleep over it. Obviously lubing is important, and cleaning up too. You can hose down the big grit (unless you're very concerned about grit going deeper into your cogs, but I find this to be less of a problem than a very dirty bike) and make sure you reapply lube whenever it's been wet. Some lubes are better than others for the wet weather but if it's not a frequent event for you it doesn't matter that much. I also inspect my tires more because rain tends to bring more grit on the road.

Rain gear is important, especially in cooler weather, so you don't get cold. Wool is great.

But for a one-time thing... just go and ride, and laugh!