LoadGen has been re-released and is now available for download at:
http://www.microsoft.com/downloads/details.aspx?FamilyId=C2EE632B-41C2-42B4-B865-34077F483C9E&displaylang=en

If you had installed any previous versions of LoadGen, uninstall these first and then install the newly updated version. 

LoadGen can be used to generate load against BizTalk Server for performance and stress testing purposes.  This kind of testing should be part of every BizTalk implementation's pre-production work and this is a fantastic tool to use for the job.  In fact, our internal testing teams use exactly this tool for our own testing.  Note: This tool should be used in a test environment only, and should not be used against a production environment. This tool is provided "as-is" and is not supported.

LoadGen requires the .NET Framework 2.0 and the .NET Framework 2.0 Software Development Kit (SDK) on the box, so make sure these are installed before LoadGen.  Once installed, the application can be run from the Install Directory/Bins folder where you will find LoadGenConsole.exe.  This takes as input an XML configuration file which specifies the load profile to be created.  The documentation is quite comprehensive in this area so be sure to give it a read.  There are also samples included which are worth a look.  There is the notion of "Sections" within this configuration file and it's not obvious, but these are run in parallel with each other.  This allows you to create intersecting load patterns.  Something to keep in mind is that generating load does not come for free.  Be sure to keep a close handle on your system resources during these tests and determine if it's more appropriate to have LoadGen run on another machine altogether, isolating the performance impact as much as possible.  LoadGen supports a number of the native BizTalk adapters and has extensibility options to allow for creation of your own adapter harnesses.

Another important point to bring up is use of this tool with MSMQ.  This is supported, but we do not auto-register the MSMQ COM components during installation since the MSMQ runtime service may not be installed on everyone's machine.  To use MSMQ and LoadGen, manually register the MSMQTransmitter.dll and ComMsmqMonitor.dll files located in the Bins directory, as shown:
> regsvr32 MSMQTransmitter.dll
If you do not, you will receive the following runtime errors:
"Cannot Load Transport DLL C:\Program Files\LoadGen\Bins\MSMQTransport.dll for Section MSMQRxQTxn
Exception has been thrown by the target of an invocation."

Here's a helpful formula to assist with constructing your LG config files:

Happy testing,
- Doug