In trying to resolve an issue with regards to computers loosing thier trust relationship with the domain, I found this increadibly good explaination of how passwords are used and changed in AD. It goes a long way to explain where is the password change is initiated (the computer, not a DC), what is required to be in place before the change can happen and how it works around an issues it may face. Thanks Manish Singh (from the Directory Services team) for such a detailed post. Added to links too!
Powershell for Active Directory