While I agree with the rest of your post, I want to comment on your last point:

One should never assume the clients are secure. Always (if at all possible) treat clients with utmost suspicion, and that means not trusting any data you get from the clients unless it checks with the stuff you know. Taint checking all the input should go without saying.

Assuming the client is out to get you not only protects you from clients that have been taken over by black hats, but also from bugs in the client...