I wonder why they decided they really wanted to use the JVM (not saying it doesn't make sense, its just its not mentioned in that presentation).
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.