How to unbind from active directory while preserving a user account?
Login to a different admin account. 
Delete your account, choosing "Don't change the home folder" option. 
Open Terminal and rename your home folder, eliminating the " (Deleted)" part -- sudo mv "/Users/yourshortname (Deleted)" /Users/yourshortname
Unbind your machine (Login Options tab, at the bottom)
Create a new account with the same short name.
Use the existing folder, when you're prompted.


An update to my comment of Sep 6 at 15:24 and part of #4 -- Apple has just issued a supplemental update that fixes this loophole. The text of their security announcement is:


  APPLE-SA-2013-10-03-1 OS X v10.8.5 Supplemental Update
  
  OS X v10.8.5 Supplemental Update is now available and addresses the
  following:
  
  Directory Services Available for:  OS X Mountain Lion v10.8 to v10.8.5
  Impact:  A local user may modify Directory Services records with
  system privileges 
  
  Description:  A logic issue existed in Directory
  Services's verification of authentication credentials allowing a local
  attacker to bypass password validation. The issue was addressed
  through improved credential validation. CVE-ID CVE-2013-5163 : the
  rookies of 42