Why won't OS X Mountain Lion remember smb password on login?
You can create a link file with the username and password, and put that in the login items.

In Finder select Connect to Server and enter:

smb://domain\username:password@serveraddress


Don't click connect or press enter, just highlight the whole address and drag to desktop (or anywhere else), and it will save as a file. You can rename it, if you like.

Then from System Preferences, Users and Groups, Login items, add the file you just created.