Hi Dmitry, inspired by your post I've put together a small Rails plugin
that lets you create unobtrusive AJAX validations that hook into your
model validations, making things nice and dry. If it finds an error
when you tab out of a field, it adds an "error" class to the field and
inserts a list of validation errors. It can also take a callback which
it sends the errors to in JSON format for advanced handling.
There is still some cleaning up to be done but I hope to have something
out next week.