Importing from a Windows 2000 server, I have an NT user, let's call him 
"Jonathan Harker" and his username is "JHarker".

I run "net rpc vampire" to import the users.  My "add user script" is NOT
called for user "JHarker" because I already have a Unix account named "jharker"
(i.e. the Unix account is all lower case, the NT username is in mixed case).

When the user tries to log in, they get an error message.  If I change the 
/etc/passwd entry to "JHarker", they can.  Running "pdbedit -v jharker" shows:

   NT username:   JHarker
   Unix username: JHarker

I didn't want to use mixed case usernames in /etc/password, so I fired up emacs 
on passdb.tdb and manually lower-cased the three instances of "JHarker" in the 
file, and that worked too.