For SLR I used small black Roots backpacks (these are padded with inserts and very simple, not the digital monstrosity with 15 pouches). They are very comfortable,strong fabric, unobstrusive and the inserts make it convenient to store and grab once you open partially the bag.
If you are interested but cannot find one (they are discontinued), I still have two and I could part with one since I have sold all my 35mm gear.
Domke are also very nice (I have many) but I find that messenger type bags are more tiring for the back after an hour.
I went with a Lowepro Event Messenger 100. It isn't as pretty as the Domke, but it also doesn't cost more than the camera body its going to protect. :D
And its about the size I had in mind.