How do you make hint text like Mobile Me?

Could anybody tell me how to create something like the hint/labels on Apple's MobileMe login page (www.me.com) in that the label/开发者_如何学JAVAhint text just fades when you click on the box and doesn't go away until the user starts typing?

(And if you could use JQuery to do it, it'd be even better.)

Thank you very much!

(By label/hint text I mean the gray 'Username' and 'Password' text.)

The effect is called a "watermark" and there are plenty of JQuery plugins to help out:
http://plugins.jquery.com/search/node/watermark