The D70 should be pretty tough. I've got an old EOS 5 which is built to a similar standard, and I've bounced it off rock and concrete a couple of times to no effect. It's usually mated to Canon L series zooms, so probably costs about the same as my bike.

I usually don't use anything more than a neoprene Zing bag to protect it before putting it in a pack or pannier. I've also got a Lowe Pro chest-mounted zoom bag, but it's far too bulky to pack casually.

If you're worried about landing on top it, consider a handlebar bag. Seriously, when I carry my camera gear in a backpack in a precarious situation, I'd be more worried about hurting myself if I fell on it.