If you want to change the default sender used by WordPress if it cannot determine a proper sender name, you have to look in the file wp-includes/pluggable.php, lines 352 and 369 (at least in version 2.8.2):
line 352:
$from_name = 'putyournamehere';
line 369:
$from_email = 'putyournamehere@' . $sitename;
As this means changing a WordPress standard file, this may be overwritten when you upgrade WordPress.
