I think it's not really answering your question but here is a free extension that allow you to use username instead of email. Well exactly this extension allow you tu use both field as credential. (sorry for my english...)

https://www.magentocommerce.com/magento-connect/username-support-login-register-checkout-by-diglin.html

Hope it will help you !

I think it's not really answering your question but here is a free extension that allow you to use username instead of email. Well exactly this extension allow you tu use both field as credential. (sorry for my english...)

https://www.magentocommerce.com/magento-connect/username-support-login-register-checkout-by-diglin.html