Nice to have a full-blown client-side equivalent of schema definitions and validation rules, but for most of my forms, I just add it quickly and dirty. Sometimes it feels bad, but most web toolkits treat JavaScript as a second-class citizen and not an integrated part of its core (and no, throwing in a jQuery does not count).

Yii framework (PHP) supports validation through Ajax (serializes the form, sends for regular server-side validation and displays the errors on the form). A godsend if your form does not change, and impossible to work with if it is dynamic (consider batch entries with "Add line" features, etc.).

I haven't seen a seamless integration on this higher level.

Yii can also do client side validation (without an ajax call) for most of its built in validation rules.

Is the form definition-to-JavaScript code open source, by any chance?

I ported django.forms to JavaScript so I could use it with Node.js, but it also runs on the client: https://github.com/insin/newforms

Could be useful to plug the two together.

With Rails I did come up with a similar solution to Parley.js. Just add a helper that renders the needed data-attributes and you're done.

Your solution sounds nice for single page apps.