Commuting - Leave my bike in the rain?

I want to know how bad it is for my bike if I leave it parked outside in the rain. Should I cover it with something or will it be ok on its own? Also, what if any maintenence should I be doing when I get home after a rain ride? I use a Trek 4500 as my commuter.

I had to leave my bike parked outside for 2 years, in a coastal town. Its not a big problem for a commuter bike. Cover the saddle with a plastic bag (the supermarket ones without ventilation holes).

To weatherproof/winterise a bike, clean the bike frame well and let it dry. Put a couple of layers of car wax on.
Wax the exposed cables.
Put plenty of grease inside any cable outers.
Spray some WD-40 down the seat tube and if possible into the small pinholes at the ends of your seat and chain stays.
Smear grease (or copper antiseize) on seatpost inside the tube. With a threaded stem, grease the quill stem inside the steerer. Im not sure how to protect threadless systems best.
Ensure all the bolts (inc pedals) are greased.

Use a wax-in-solvent chain lube on a warm dry chain and make sure you cover the sideplates as well as the internals.

Originally posted by MichaelW
Use a wax-in-solvent chain lube on a warm dry chain and make sure you cover the sideplates as well as the internals. KMC makes a chain called the "Rustbuster" that lives up to it's name. Costs about the same as a standard chain. I've tested it and can tell you it works great if you ride in wet environments. Combined w/ the wax lube, makes a great combo.