I was recently present at an internal demonstration by Rob Jarratt of a tool to generate unit tests for WCF services from just the message trace files… and it has just been released to codeplex! The tool integrates with Visual Studio 2005 or 2008, and allows creation of a unit test that replays the messages found in the trace file. You can then use the Visual Studio test tools to apply this as load to perform scalability testing of your service layer. There are some neat little features that allow you to edit the values used for each message too to get a more realistic set of messages.
I’d highly recommend checking it out if you’re building an application that uses WCF.