Nginx HTTP Push Module seems to much better solution than EventMachine. So Nginx keeps opened sockets and a Rails app sends messages via ordinary HTTP.
I'm also wary of handling websockets from Ruby, which is demonstrably ill suited for any kind of high concurrency networking (been there, got the t-shirt).
Since most people already run nginx in their stack anyway I don't see the advantage of sticking with a Ruby-based solution that is almost guaranteed to be inferior on every metric.