Well at least I'm not the only one with this issue.
Both of my DL380G6 hosts are running ESXi 4.0.0 release 208167 which I believe is the Update 1 build.
What has me most confused is that my 2 hosts are basically identical. The server hardware is the same and all NICs are connected to the same switch, yet only one of the NICs comes up as 100mbps