Really, what's the difference between scanning the message in order to, say, render HTML tags it may contain, and scanning it in order to generate targetted advertising based on keywords it contains?

That's irrelevant. These arguments that Gmail is just like other services are nothing but red herrings. Surely cypherpunks should understand that the recipient of a message can do whatever he wants with it. He can save it to disk, he can share it with his friends, or he can contract with Gmail to add targeted advertising. It's contrary to everything cypherpunks stand for to suggest that the sender of a message should have some power or authority over what happens to it once it is in the receiver's hands. Even if Gmail were completely new and nothing like it had ever existed in the world before, it would be perfectly acceptable for mail recipients to use the service. That follows from their inherent freedom to use the information under their control. Please save these tired arguments by analogy for another forum. Cypherpunks agree on the basic desirability of individual freedom, and that is enough to settle the question.