Not THE answer, but here is one guys idea. Sycip Custom Bicycle Messenger Bike.

I like my bike for this app: Cro-Moly frame & fork, road bike derailleurs (Shimano whatever), mountain bike headset and tires that can do well on pavement because you'll mostly ride pavement, but not so narrow like dedicated road bike tires. I like the Slickasaurus, but there are other tires that also fit the job description -- wide enough but easy rolling.

When I first rode my bike in Alexandria, VA I got on those cobblestones on Prince Street near Union and that sold me on the wide slicks. Dedicated road tires don't do well with ruts, train tracks and other road hazards. Wider tires are much more forgiving IMO.