I carry a lightweight raincoat that is waterproof and has a hood large enough to go over my helment. In cold rain, a hood will make an enourmous difference in your ability to keep warm. My favorite jacket also has pit zippers to help with the air flow.
Now, the dilemma that many people notice is that if you are warm enough inside your rain jacket, people often become wet with their own sweat. I find that scenario to be more comfortable to being pelted directly with rain.
Also keep in mind the regional factors. In the mountain west, there are many places where you cannot simply call it a day early unless you plan to set up your tent in a ramdom place in the woods. There may be times when you are 50 miles from any kind of indoor facility to even hang out in for a while. You'd definitely need to be prepared to keep warm in that situation if you are travelling in the Rockies or Cascades. Also bear in mind that it can snow in these mountain at any time during the summer. It would not be unusuall to wake up to a beautiful August day, have the temperature get into the 90's and only hours later find your self in a very cold rainstorm.
My point being, I recommend that every long-didtance bicycle traveller has a decent rain coat with them at all times regardless of where you travel or what time of year it is.
Adventure Cycling Association