I'd be very hesitant to strap a rack to a carbon frame.
Whatever type of mounting system you use (I'm not aware of anything but the little metal collars that substitute for braze-ons, but I'm not really up on touring gear, either) will tend to rub the frame over time (yes, they come with rubber cushions, but grit will still work its way in there). Some scratches in the paint aren't a big deal if it's metal, but I'd be seriously worried about weakening composite. (The same goes for attaching the collars to carbon stays on a bike with Ti or Al main tubes).
Plenty of roadies on the ride I just did this weekend were wearing fanny packs, especially on the second day when the weather was a little more uncertain and people were wearing jackets. IIRC those things were invented for cyclists to begin with, and it's why they're called "fanny packs" even though most people wear them in front. A moderately sized pack has room for a compact jacket, or the sleeves off a convertible jacket, arm warmers, leg/knee warmers, energy bars or gels, whatever you need. As for a laptop, is your commute too long to wear a backpack comfortably?
ETA: you would've appreciated one of the riders I saw yesterday. She was wearing a wool jersey, probably too big for her to begin with, that had stretched after getting wet, and her pockets were very full. The pockets and their contents were hanging down below the back of her saddle like a tunic 
And pepper spray goes best on a handlebar mount, they're available. If you need to spray a dog in a hurry, you don't want to be rummaging around in a pack.
Last edited by OakLeaf; 05-12-2008 at 07:26 AM.
Speed comes from what you put behind you. - Judi Ketteler