Having rode over 300 miles in the rain this year, I offer one indisputable fact: you will eventually reach a point where you cannot get any wetter than you already are.
For example, when I did the RSVP this year, it started pouring after the Canadian border. I rode 40 miles in the rain to Vancouver, and was no more miserable at the end than I was after the first mile of the deluge. Actually, as long as I kept moving, I was plenty warm.
I biked to work today, during one of our recent downpours, in order to test the effectiveness of my various rain gear. (And before you say anything: yes, most people I know don’t understand why I like biking long-ish distances in perfect weather and yes it’s okay to question my mental capacity for wanting to bike to work in the rain.)

Wear your CamelBak under your jacket. But, make sure you have a large enough jacket. A bike specific jacket with a longer tail works best. Otherwise, while you are riding leaned over, the jacket will ride up and leave a nice opening. More so with the CamelBak underneath.
“I offer one indisputable fact: you will eventually reach a point where you cannot get any wetter than you already are.”
Ain’t it the truth! If you have never ridden to this point in your life, I highly recommend it. Somehow, the ride changes from being perhaps a slog to being a joy-filled craziness, as you ride along with your mouth hanging open and tongue sticking out (better to catch the rain) and all you can do is laugh.
I am about to go biking in the rain right now. Found your site by googling “biking in the rain.” Well done!