How can I change the text colour and font of the username and password in Lion login
Apple's ideology is mostly to not let the user tweak the design because, in many cases, the user may choose inferior fonts, colors, etc, and thus make Apple production look bad.

Instead of forcing design choices on user, as proudly does Linux and often does Windows, OS X contains settings that Apple considers “best” for everyone.   

Of course, some people complain about certain decisions but it's much like MySpace vs Facebook. You can't change fonts and colors on a Facebook page, but frankly, imagine what a mess it would be if anyone could. 

Presumably for this reasons, you can't customize how the text looks in input fields, including the login screen.