vlan on em0 cannot set MAC address.

[Zaphod Beeblebrox]

I have a FreeBSD-8.3 machine with an em0 interface in it.

em0: <Intel(R) PRO/1000 Legacy Network Connection 1.0.4> port 0xd400-0xd43f
mem 0xcffa0000-0xcffbffff,0xcff80000-0xcff9ffff irq 12 at device 17.0 on
pci0
em0: [FILTER]
em0: Ethernet address: 00:0e:0c:bc:6f:87

For various reasons, I have more than one DSL interface, and for some time
I have run two PPPoE connections (using mpd4) and I run those PPPoE
connections over VLANs on the em0 interface.  There are two switches on the
path and both switches are sufficiently competent to understand that two
different VLANs include the same MAC address.

I'm convinced that I used to be connected to two separate DSLAM devices and
that after some line trouble, I was reassigned to two ports on a single
device.  This is because using the SAME MAC address on both VLANs stopped
working.

To fix this, I tried setting the MAC address on one of the VLANs.  This
does not work.  em0 no longer transmits the packet.

Expected behavior?  Are there drivers that support different MAC addresses
for different VLAN sub-devices?