Nope, but there is no reason to think that a sufficiently motivated and well funded party wouldn't be able to pull it off. If governments can build things like stuxnet then a project like this should be relatively straightforward.
It should be the default because if only those people who need the protection use crypto, they stick out like a sore thumb, negating much of the protection, if not actually increasing the risk. If only political activists (say) use GPG, then the easiest way to get rid of the opposition in a country is to round up everyone that's been seen by deep packet inspection machines sending a GPG-encrypted message, which is rather trivial to automate.
Also, you just don't have a clue who is watching your traffic and what they are using it for, and machines are only getting more powerful, enabling ever more advanced analysis of your communication (think of someone intelligent, with a brain, watching everything you do, or rather watching everything everyone does, but with sufficient intelligence to pay as much attention to you as a single person could watching a single person - that's probably not an accurate model (yet), but still probably closer than what you imagined). Imagine a representative from your internet provider or the government ringing at your door - if you wouldn't let them in to sit next to you/follow you whereever you go around the clock, you probably would also prefer encrypted communication if you understood what one can do with your internet traffic.
Well there is very little cost for what it offers. It takes developers a few days or more likely now a few hours to setup and only serves the viewer better. It affords the visitor some level of trust that the site hasn't been tampered with and their login credentials aren't being siphoned off for example.
If you are the author of a blog (with comments disabled), and you don't care if your message is manipulated, then thats your choice. But before you know it vanilla http will be blocked in browsers, and you'll need to make the change anyway.
