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.