Many people are in the process of upgrading from 100MBps to Gigabit (I've got a few machines Gigabit capable at home, but am still running almost exclusively 100MBps except for my iSCSI storage device). I noticed there's a KB article on microsoft.com which documents a couple of solutions to slow performance problems between guests and host in terms of networking, especially when using Gigabit network adapters. To be honest, I haven't noticed any problems, but that could be because I'm running recent builds of Virtual Server 2005 R2 - soon to be launched, just a few days more to wait 🙂
Note that disabling offloading on the network adapter of the host is also one of the most common causes of problems with VSMT when capturing and deploying images. The KB gives a little insight into why that might be the case - "Note We recommend that you start with the Offload TCP Segmentation property. The problem is probably caused by the Offload TCP Segmentation property because full support for this feature is not implemented in the Vpcnets2.sys driver." Maybe they fixed it in MSVS R2, or I've just been lucky!