I've been trying to track down some performance issues with one of our apps that started when I moved the file share the app uses from Windows 2003 to Windows 2008.
One test I ran was TTCP in various combinations of 2003/2008 to see if there was a networking problem. For those not familiar with TTCP, you run it on two hosts and it sends traffic between the two hosts. It doesn't write anything to disk, so it's a pure network test between two points.
It appears that at least in some directions, 2008 is hosing things up. Especially the 2003-to-2008. These are all VMs on ESXi 5.1. I'm not sure if this is a networking issue, VMWare, or Windows. What would you check next?