I've been using Jerkson (a layer on top of the excellent Jackson JSON processing library) to do case class marshalling/unmarshalling. It works quite well - in fact, nearly perfectly, but it still uses runtime type inspection to get things done. A nice, simplistic typesafe compiled parser would be ideal.
I'd love it if the play folks would break this out of the core library - it seems like this would be applicable to the whole Scala ecosystem, and not just Play.
There are a few github forks that have added a couple useful things. For my use case (converting case classes to/from JSON in scala 2.9.2) it works perfectly.
I've also written a Jackson plugin that knows how to marshal POSO classes - it's just an introspector that uses the scala convention instead of the JavaBeans convention. By and large it works well - perhaps I should put it up on github.
I'd love it if the play folks would break this out of the core library - it seems like this would be applicable to the whole Scala ecosystem, and not just Play.