Yammer's productivity gains from Scala were apparently not greater than the losses due to poor tooling and the impedance mismatch between Java/JVM and Scala. Therefore, the engineers all thought it was better to just go back to Java.
That seems like a useful data point, but certainly others' experiences might vary. Have these other posts added any extra information?
If anything, I just find it really odd that a company feels the need to have an official statement on what programming language they use/will use/might use.
Many people (including some people on HN) started criticizing Yammer because they decided to go back to Java. This blog post was in some sense necessary for Yammer.
He didn't want the last word to be Yammer hates Scala and need's to jump ship. He thought it necessary to give a more balanced overview and provide subtext to the original message. I think he succeeded in doing this.