I'd like to agree with both Doug and Europanorama. I've travelled tens of thousands of miles on motorcycles, with cameras -- probably well over 100,000 miles, in Europe, the USA and India -- and it's a bloody harsh environment for a camera. The foam-plus-tank-bag route is the only one I'd recommend: I've had the back fall off a Hasselblad and an internal lens element unscrew in a Vivitar Series 1 200/3. Nowadays I take only seriously tough, simple cameras on 'bikes. The swing on a swing-lens is just one more thing to go wrong.

Whatever you take, Zip-Loc bags around the camera(s) will help keep out dust from the road and fluff from anything else in the bag. You might want to take a look at www.mctie.com, aka www.motorcycletouringineurope.com (either link will work).