Hi Ben,
I remember from a different thread on MDIO that you were posting to that mentioned you had your own linux distro that is different from the distro provided with the SDK for the AM335x. From what you are describing the cpsw interface is not being initialized.
There is a function called am33xx_cpsw_init in the file devices.c that was mentioned in an earlier post in this thread. Is this function being called? Or perhaps just this function below?
pdev = omap_device_build("cpsw", -1, oh, &am33xx_cpsw_pdata,
sizeof(am33xx_cpsw_pdata), NULL, 0, 0);