Ryan Troy (Ryan Troy) asked that I share this in case someone happens to run into it.
I had a scenario with a customer using brand new supermicro servers with Broadcom BMC57810 10g nics. They were having network performance issues to the point the interfaces were only performing around 10-30mb/s. The issue ended up being old bnx2 drivers (version 1.72.54.v41.1) present in ESX 4.1u3, I'm not sure if this applies to ESXi 5.x they didn't have a server to test on. The customer updated the drivers to (1.74.22.v41.1) and the performance is now normal.
Drivers as of 1/13/2013 can be found here.
In ESX 4.1 you can find the driver version by running these commands via the CLI.
Grab the nic device name
Name PCI Driver Link Speed Duplex MAC Address MTU Description
vmnic0 0000:05:00.00 bnx2x Up 10000Mbps Full 00:25:90:88:ed:ac 9000 Broadcom Corporation NetXtreme II BCM57810 10 Gigabit Ethernet
vmnic1 0000:05:00.01 bnx2x Up 10000Mbps Full 00:25:90:88:ed:ae 9000 Broadcom Corporation NetXtreme II BCM57810 10 Gigabit Ethernet
Grab the driver version
#ethtool -i vmnic0
firmware-version: bc 7.4.19