Upon update 1.5 you cannot use the email field to populate the username field!

Josh, did you get this working? I also liked that feature. I am not sure how to implement the workaround.

I have modified ABT's code above, and made a few improvements:

1. It looks for the email field by GF type, instead of label.
2. When it finds the type, it verifies that it is a proper email address.
3. It verifies that the email matches the confirmation value.
4. It verifies that the "username" (re: email) is not already registered.

It seems that my validation message for #4 isn't being displayed (GF is overriding it with "Username is already registered"), but it serves my purpose, so I haven't investigated why.

The setup of my form in GF is as follows:

• no username field
• email address field with confirmation enabled
• user registration: create user maps both username + email address to the email form field

I hope anyone who stumbles upon this finds it useful. I imagine that a LOT of people are trying to do this. GF should really add a solution to the documentation!

// allow user to register with email address by stripping validation errors on
// the email address, so we need to verify that it meets the following conditions:
// 1. Proper email address
// 2. Email address matches confirmation value
// 3. Email address is not already registered
add_filter( 'gform_user_registration_validation', 'custom_filter_registration', 10, 3 );
function custom_filter_registration( $form, $config, $pagenum )
{
	foreach( $form['fields'] as $k => $field )
	{
		if( strtolower($field['type']) == 'email' )
		{
			// get email address + confirmation value
			$id = $field['id'];
			$email_1 = strtolower($_POST['input_' . $id]);
			$email_2 = strtolower($_POST['input_' . $id . '_2']);
			// verify: proper email, and email matches confirmation
			if( is_valid_email($email_1) && $email_1 == $email_2 )
			{
				// verify: email is not already used
				if( !function_exists('username_exists') )
					require_once(ABSPATH . WPINC . "/registration.php");
				if( username_exists($email_1) )
				{
					$field['failed_validation'] = true;
					$field['validation_message'] = 'The email ' . $email_1 . ' is already registered.';
				} else {
					// OK: remove validation errors
					$field['failed_validation'] = false;
					$field['validation_message'] = '';
					$form['fields'][ $k ] = $field;
				}
			}
		}
	}
	return $form;
}

function is_valid_email($email) {
	return preg_match("/^[_a-z0-9-]+(\.[_a-z0-9+-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,})$/i", $email);
}