In order to use some of Google's features, especially when modifying a user's data, you must use their authentication scheme. Currently, they only provide an authentication mechanism for desktop/client-side applications. But their site says that a web-based authentication proxy is coming in late April, which is... oh, about now. :)
Account Authentication for Installed Applications: The ClientLogin API lets you use programmatic login in your desktop or mobile applications. This API incorporates CAPTCHA for greater security against password trollers.