This just in.  Today the Doculabs "@Bench" Web Services Benchmark came out.  Here's a snippet from the Table of Contents to give you an idea of the scenarios they tried:

Summary Results: Application Server X on Windows 2000 and Oracle9i
Summary Results: Application Server Y on Windows 2000 and Oracle9i
Summary Results: JBoss on Red Hat Linux 8.0 and Oracle9i
Summary Results: .NET 1.1 on Windows 2000 and Oracle9i
Summary Results: .NET 1.1 on Windows 2003 and Oracle9i
Summary Results: .NET 1.1 on Windows 2003 and SQL Server 2000

The results you ask?  Let's just summarize by saying:

• Windows and the .NET Framework is the Best Platform for Web Services
• JBoss on Linux was the fastest Java system (!)
• Web Services on J2EE offers inconsistant performance
• Oracle on Windows 2003 Server is a very viable option
• Windows 2003 Server and Oracle offered a 37% increase in throughput compared to Windows 2000 and Oracle
• Windows 2003 Server and SQL2k offered roughtly an 38% increase in throughput compared to Windows 2003 and Oracle