I should add/clarify that if you're talking about dynamic grains (active discussions, wikis, etc, as opposed to mere statically published sites), it is also alr

I should add/clarify that if you're talking about dynamic grains (active discussions, wikis, etc, as opposed to mere statically published sites), it is also already possible to share those publicly. They're just at weird looking URLs. Hence the interest in a "well-known subdomain" feature to improve that.