My contention is that if you want to, you can make ads nearly indistinguishable from "content".
On the extreme side, I would say that I have won this bet already; if you pick up something like the Wall Street Journal and count the number of articles that someone involved wanted printed, you'd find that most of what you're reading is one kind of ad or another.
But there are technical solutions as well. Obfuscation beats machine parsers for at least the foreseeable future.
