So, you've installed your infiniband cards, but got limited performance improvement for your MSMPI application. Before you look at the code again, you may want to have a look at whether the winsock direct providers were installed as well. In some cases...

iWarp (aka RDMA support, TCP offload and kernel bypass on Ethernet) will be for some a viable alternative to Infiniband as a cluster link, especially on the new 10 GbE adapters. Microsoft's MPI will make use of it, as long as a Winsock direct provider...

It's OUT!!! ... and we have announced that we shall support VMWare, Xen in a future release. I cannot believe it! :-) Check out http://blogs.technet.com/virtualization/archive/2007/09/06/system-center-virtual-machine-manager-huge-announcements.aspx