I use a small Topeak handle bar bag:
http://www.topeak.com/2007/products/...ndlebarbag.php. It is nicely padded on the inside, is small and light enough to not affect steering and comes with its own rain cover. Those designers at Topeak are amazing.

I used to use a rear rack and bag, but I spent too much time twisiting round to get the camera. This Topeak bag fit my camera perfectly and would have room for a lens or two in its second compartment. With the camera in front I was more likely to stop, pull it out, and grab the shot I wanted than with it in the back.

The bag converts to a fanny pack off bike, but I put a shoulder strap on it instead. It is just me, but I don't think I'd feel comfortable with a shoulder bag. The handlebar bag has nice padding and, if I were to fall, the camera and I will survive much better if it is tucked away in the handlebar bag. But, I'm a klutz.