Fitts’s Law basically says that if you make something bigger and more clickable, it is easier to click on. Terrible truism but ultimate proof that common sense isn’t.
If you want to get Fitts’s Law, or explain it to a client that doesn’t quite understand, send them to this post.
Word up to Daring Fireball for the nudge.
I was researching an article for PortaBlogger and came across this web form validation/confirmation mechanism on the Opera Community site:

As the form fields are filled in, the green check icon appears for a valid entry - in real time. Here’s an example - I entered the first four letters (Andr) of my desired username (AndrewBoyd), and saw this:

Isn’t that cool? I don’t have to fill out the whole form then wait for 30 seconds while it decides whether my state should be Alabama or ACT before it tells me that I need to choose another user ID. Not only that, it suggests alternates on the fly without getting in the way.
I’m working on a project with Matthew Hodgson at the moment where we’re having to look at intelligent data entry validation mechanisms, so this is very timely.
I can’t fault it - if you would, go to the Opera Community site and have a play with it - if you can find a hole in this as the friendliest form validation ever, please let me know.