Hello,
I've implemented this sometime back and have experienced the following.
On HP or Cisco switches if I do any maintenance where I turn off port authentication and turn it back on some of the phones try to authenticate to my ACS box with usernames that I have not programmed into the phone.
The setup we choose to use is create a username and pw for each location and manually enter those into the phones because folks move around within sites vs locking them down via MAC address. I've brought Mitel in on this and they claim they have done extensive testing and couldn't duplicate the issue. Those usernames are getting generated somehow and thought maybe somebody else has had this issue. The usernames are jibberish like "...WMT" the . is included
Thanks,