Almost forgot to check in:

OpenSim.32BitLaunch.exe is a 32-bit application that loads OpenSim.exe using .Net framework, hence it is JIT-compiled to 32-bit.
Use this app to start OpenSim on 64-bit systems (works great on Vista 64 :))

OpenSim/Tools/OpenSim.32BitLaunch/Program.cs

@@ -0,0 +1,10 @@
+namespace OpenSim._32BitLaunch
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            OpenSim.Application.Main(args);
+        }
+    }
+}