New user registration - make username and display name the same

[Cache-man]

It would seem that it still confuses people to have 2 different name fields on the user registration form.

I want to know if it is possible to somehow edit the my registration form so that new members only have to fill out user name (display name field to be hidden), and their username automatically gets copied into the display name field.

Is this possible?
If so, which file should I edit, and what code should I use.

Oh, by the way I am using the cowboy bridge to bridge mambo and smf.