Which requires active attacks, ie MITM. And stopping MITM is essentially impossible - the best you can do is use our current CA setup, which assumes the original download of the certificate you got wasn't hijacked.

In preventing passive listeners, like Firesheep, this would work 100% effectively anywhere it can work at all. The only way it reduces security is in making people who don't understand MITM attacks feel they're safer than they are - at absolute worst it's like you don't have it installed.

I hadn't thought about that. However, at the bottom of tcpcrypt.org, it said "You're not using tcpcrypt =(" to me, so perhaps this could be used to show a (less obtrusive and less intimidating than browsers do when they detect the lack of SSL for some content on the page) warning to users that they are not using Tcpcrypt.