Thanks for that Crumpler link! Looks great.

I carry my DSLR in a Topeak handlebar bag. This one: http://www.topeak.com/products/Bags/...deHandlebarBag
It comes with a rain cover that has never failed me, and I also carry a freezer ziplock bag for when it's really coming down. Another smaller one for batteries and cards.

I did take a proper photo knapsack once and strapped it to the back rack, but it was too hard to get into during the day.

If you get a good quality bag I really wouldn't worry about the plastic bracket. Just make sure not to carry way over the reasonable weight limit, but camera with one lens will be just fine.