Correct me if I'm wrong, but isn't the mac address more or less the lowest level of addressing on an ethernet network? So, the computers on network have to know which computer has which mac address (or rather, the routing tables contain this info and if it isn't there the computer sends a broadcast request) so that it can know where to send data. Therefore you have to have the correct mac to communicate, so there isn't any way you can "hide" a mac address, someone with ethereal will always be able to see where an individual frame came from and where it's going to mac-address wise.
Again, networking isn't my strongest point, so I may very well be wrong; corrections welcome.