$("#my-form input[@type='text']").blur(function() {
var query = {};
query[this.name] = this.value;
$.post("/path/to/your/validation.php", query, function(response) {
// do something like changing a css class, disable submit...
)};
});

http://beta.plazes.com/register/

Found on the jquery mailing list: http://jquery.com/discuss/2006-April/004625/
The url shows it in action, checking text when you move away from a field.

Use jQuery to validate form text

form, jquery

JavaScript