1982 Lotus Legend (steel-frame touring bike); 1982 Fuji S10S (converted to a singlespeed: 46x16); Specialized Crossroads hybrid (the child taxi).
Is your bike a nice lugged-steel vintage Serotta? Or do you ride an aluminum mountain bike? Generally, when I've had to leave a bike in the rain, I leave the steel bikes at home and make sure that I tie a plastic bag around the seat. Wipe it down when you're back home. It should be fine.
70s Leader Precision w/Shimano 600 (road), IRO Rob Roy (Fixed)
Whether you leave your bike out in the rain or ride it in or after rain, it's going to get wet. Lube/maintain your bikes mechanical parts so that they're prepared for it, get a saddle cover for when you park outside (a plastic bag works just fine for my Brooks) and that'll be it, I'd say.
Though I don't like to leave my bike outside in the rain, I know it's not going to be a big deal - all I make sure I do is (as with every lockup) remove all my lights/computer junk. And, of course, the bag over my Brooks saddle.
Constant exposure to sun and rain is hard on a number of components but if you are using the bike regularly and you keep up on the maintenance, including lubing all the bare metal parts, it should last for a long time. Boeshield T9 claims to be designed by the aircraft industry for just this type of protection.
I ride my bike every work day, rain or shine. At work it gets to be parked inside the garage where its nice and dry, but at home I have to leave it out. I just make sure that I cover my brooks saddle. I use a shower cap.
I also clean and lube the thing every weekend. In the winter I use a dry teflon lube. In the summer I use White Lightening.
Like everyone says, regular lubrication and a plastic bag over the saddle on wet days are all most bikes need. After all, what good is a commuter that no-one can commute on because the weather is too precious?