Alex Payne's Why Scala at C4[2]
slideshare.net
slideshare.net
Slide 25 hints at this, but I seem to recall Alex saying they used the Java-based Apache HttpClient in their test cases.
For example, a crashing/security bug like the one that bit Ruby in June is in much less likely for the JVM. And, there are lots of ways to profile the memory usage and code/threading performance of Java apps.