The “Log In” string is passed to the translation function, and you can filter that. To prevent needing to filter every little string, you can activate this filter right before the login form is printed.
add_action( 'login_form', 'wpse17709_login_form' );
function wpse17709_login_form()
{
add_filter( 'gettext', 'wpse17709_gettext', 10, 2 );
}
function wpse17709_gettext( $translation, $text )
{
if ( 'Log In' == $text ) {
return 'WPSE 17709 Logging In!';
}
return $translation;
}