When bonding two or more interfaces, 'ifenslave' does not set the HWADDR for any additional NIC properly. You must use ifconfig to set it manually.
This happened on a Dell 6400 with Intel KodiakII Gigabit cards (2) with RedHat 6.2 running 2.2.14-6.1.1 (sorry for the mis-labeling 7.0). This seems to have been fixed in 7.0.