The RSS feed system is password protected in order to protect your data. Note that it accepts the commonly used RSS password header ... however, Google reader in particular does not support password protected RSS feeds.
If you're attempting to access the feed from a script ( like in PHP or Python ) some RSS libraries will allow you to access password protected RSS feeds. For those that don't, you may be able to use a workaround like cURL with the cookie jar option to fake a login and authorized request to the RSS feed for your account.