i have a rediculous amount of toil and tears put into this problem, i lived on the northside/regent sq and worked for a photographer on the southside bringing all my gear back and forth by bike almost every day. i would highly recommend using a handlebar bag if you don't want to carry it on your back but i ended up doing this in the end:
I bought the ortlieb hip pack after looking around a whole lot, and successfully using topeak handlebar bag the whole time, but just not happy with the amount of vibration my gear took every day. ortlieb makes a camera insert for another bag that fits perfectly in it (they designed it like this, its not a fluke, their customer service are the ones that suggested this setup) and it adds some really nice padding (and compartments!). there are lots of ways you can wear this bag, and it is very comfortable. there is even a outside pocket for dry days that perfectly stores a medium zoom lens. for days i needed all my gear i would wear my messenger bag like normal and either wear the hip pack over my shoulder (with a compression strap even) like the messenger bag just right on top of it, it is suprisingly comfortable. you can also wear it around your waist below the bag. i just got done with a 6 month sailing trip and used the bag the whole time... including dropping the hip pack into the ocean twice, falling in with it on, and lots of other very wet mishaps not a drop of water got inside the bag. i would definately reccommend it, its even better for days when you just need 3-4 lenses and your camera, the bag has tons of space and can easily accomodate a day trip. if you buy it and don't like it i'll buy it from you.
i got everything for 110 shipped i think, the camera insert is like the third thing down on this link:
http://www.google.com/products?client=opera&rls=en&q=ortlieb+hip+pack&sourceid=opera&oe=utf-8&um=1&ie=UTF-8&ei=MLoASqjtOab0Mu-90OYH&sa=X&oi=product_result_group&resnum=1&ct=title
official specs pdf:
http://www.ortliebusa.com/cartgenie/images/pdf/hippack.pdf