[LUNI] MAC address spoofing

[zfh]

I have been playing with IP address spoofing on
Scientific Linux (a distribution similar to Red Hat
Enterprise).  I can use ifconfig to bring the
interface down and change the mac address. (I was
using a MAC address from another system on my network,
which I brought up to see the MAC address and then
shut down before trying the spoofing.)  When I bring
the interface back up, you would think it would grab a
different IP address from the network DHCP.  Instead,
it either keeps the old IP address (which doesn't
work), or it doesn't get an IP address at all.  When I
restart, the real MAC address comes back and all works
fine.  Am I missing something?