Codex

Plugin API/Filter Reference/login message

The "login_message" filter is used to filter the message displayed on the WordPress Log In page above the Log In form. This filter can return HTML markup.

A plugin can register as a content filter with the code:

add_filter("login_message","plugin_function_name");

Example 

function the_login_message( $message ) {
    if ( empty($message) ){
        return "<p>Welcome to this site. Please log in to continue</p>";
    } else {
        return $message;
    }
}
add_filter( 'login_message', 'the_login_message' );

See Also

http://codex.wordpress.org/Plugin_API#Create_a_Filter_Function

External Resources

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Codex Resources