Our team has been able to come up to speed on Scala and get productive pretty quickly. Within a month, the team felt comfortable in Scala. Most team members read through Odersky's "Programming in Scala" book to get an overview of the language which was helpful. We've also had some team members go through the Coursera class.
Our experience is that there is a cost to Scala when it comes to new hires. Very few new hires have any meaningful Scala experience so they have to come up to speed on the language whereas with Java or PHP most new hires are already proficient in the language. In the end, however, we believe that the benefits of Scala, Play and Akka over the long term will outweigh the short term cost of training new hires. Even with our new hires, we find they are pretty comfortable and productive after a month of learning and using Scala.