I am having similar problems.

I have Ubuntu 10.04, which shares file systems using Samba. My Windows 7 client is able to read files at only 1.5Mbps. A Windows-XP client Virtual Machine is able to read files at around 50Mbps (i.e. using Samba, but not using the network adaptor).

I have a "Broadcom Corporation NetXtreme BCM5751 Gigabit Ethernet PCI Express (rev 21)", so not the low power version of the chip described in some of the posts above.