I've noticed recently a trend towards validating fields when you leave the field, so the form will immediately let you know whether your email address was valid or your username is unique. Obviously some dependent validations can't be done in this way, but in general terms it seems like a pretty good default approach to form validation.
What do you think? Should validations be performed ASAP or should they wait for a form submission? Obviously it depends - but on what? What would move you towards including or avoiding on blur field validations?