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haven't the time to do the mod, but you could do this:
create a new field 'userlogin' in the table users of varchar(25)
replace all 'username' in login.php with 'userlogin'
modify the include/user_register.php - most places (but not all) you would need to copy the username section and make a simmilar userlogin section
last you need to modify the lang_xx file and the template file profile_add_body.tpl to also allow the login name
if users are not allowed to change login name you will also need to handle that in the template - same way as this is handle to day in the template