An excellent posting covering the 'new' Microsoft BizTalk LoadGen 2007 coupled with a simple WCF test. Read all about it a the Connected Thoughts blog.
The tool in a nutshell is a command line utility called LoadGenConsole.exe that takes an XML configuration file and generates the load tests described in the configuration file. The 2007 version of LoadGen can be used to simulate load in several types of transport (file, HTTP, MQSeries, MSMQ, SOAP, WSE, WSS, WCF) and to monitor certain types of systems (file, MQSeries, MSMQ, MSMQ Large files, performance counters, SQL Server).