If you have a very long or a difficult-to-write Google username, there's a way to create up to four new usernames that could be used to login to your Google account. Just go to the settings page for Picasa Web Albums and add new usernames in the Gallery URL section. The purpose of this feature is to protect your privacy: the URLs of your albums don't have to include your Gmail username.
All usernames added at Picasa Web Albums are valid, so there's no Gmail account that uses one of them. Unfortunately, they're not aliases (you can't use them to send/receive messages) and Google doesn't offer an option to remove them. Maybe in the future Google will convert these alternate usernames to email aliases and make them more visible and easier to manage.