Interesting. I just caught my Raspberry Pi in the act of doing exactly the same thing!
It started after I installed "accountsservice" which is supposed get rid of an error in the logs during boot up. This suggested I install "gnome-control-center", which then installs a whole host of other stuff, including: network-manager and network-manager-gnome
Removing these two packages ("apt-get remove network-manager network-manager-gnome") stops the network (duh!), but then when I re-enabled the network all was good and the old hardware MAC address registered again with the router.
The versions were: network-manager-gnome (1.4.4-1) , network-manager (1.6.2-3)
If anyone know more about this, or whether this is due to some misconfiguration I had of NeworkManager (I did not see anything suspicious), let us know.