Great new content with the release of chapter 3! Is the source code for this chapter available too?
In Listing 4.2, add the line (at the top)

resolvers += "Typesafe repository" at "https://repo.typesafe.com/typesafe/releases/"

Ok, this is a personal preference but I like to use a coding style that is more familiar to non-Scala programmers.

This looks simpler
broadcastEnumerator.map(_.run(twitterClient))
broadcastEnumerator.getOrElse(Enumerator.empty[JsObject])

than
broadcastEnumerator.map { enumerator =>
    enumerator run twitterClient
}
broadcastEnumerator getOrElse {
    Enumerator.empty[JsObject]
}
Maybe it might be worth to mention what happens in the case of a slow client? You could demonstrate it with the JavaScript client going into a sleep after updating the <p>:
        function sleep(milliseconds) {
            var start = new Date().getTime();
            while (true) {
                if ((new Date().getTime() - start) > milliseconds){
                    break;
                }
            }
        }

        ...
        appendTweet(data.text);
        sleep(5000);
        ...


Not only will this just render the incoming msgs with an increasing delay, but it will also have server repercussions. The server will buffer up incoming messages (from twitter) in the Iteratee (well that is my guess here), and a slow client can blow up the TwitterStream service.

This topic can bring you to back pressure, Akka Streams and how Play (will) support them. (http://typesafe.com/blog/typesafe-announces-akka-streams) Maybe to much to handle in this chapter but hopefully it will be tackled in an advanced chapter?
Listing 2.3
========
Correction:
import play.api.Play.current
ISO
import play.api.Play.current._


To be added:
import scala.concurrent.Future


Listing 2.7
========
Correction:
val loggingIteratee = Iteratee.foreach[JsValue] { value =>
ISO
val loggingIteratee = Iteratee.foreach[JsObject]( { value =>
It puzzled me to get the Specs2 Specification example to work... figures I needed to change build.sbt:

libraryDependencies +=
"org.specs2" %% "specs2" % "1.14" % "test"
iso.

libraryDependencies +=
"org.specs2" % "specs2_2.9.1" % "1.10" % "test"

REMARK: my working version uses %%... hmm hopefully I learn later on what it actually means.