My friend Cliff posted a reference to this KB back in February - Of course it's not until you experience the problem that you really start to look at it. . We've seen a few slow "run from distribution points" as well as some slow OSD distributions, where HTTP wasn't available. If you have also, you may want to take a look at this KB:
A Windows Server 2003-based computer responds slowly to RDP connections or to SMB connections that are made from a Windows Vista-based computer
http://support.microsoft.com/kb/947773