Kickass form assistance

So have you seen Remember The Milk yet? It's a to-do list service that has some very next-gen features, including some Ajaxy stuff to speed things like form entry and validation:

Remember The Milk checks to see if the username I've selected is available as I type it and puts little checkmarks next to completed fields. I'm totally in love with this approach. How many times have you filled out a form with a kabillion fields, only to be forced to do it again because one or two fields were invalid?