Really like the look of this. It is similar to something I have been thinking about for a while. Could this object do more than just assist in nested objects? A few thoughts:

Would love for it to have a default implementation of populate for simple non-nested objects. This would just create an instance of the object being saved/validated and assign each accessor. Kind of an alternative to attr_protected or the new strong attributes.

Would be nice if validation could also be placed at this form level. This way validations at the object level could apply site-wide, but you could layer additional validation on a per-form basis. Would be nice for multi-page forms or having stricter requirements in certain setups (maybe admin screens have less validation than end user screens).

Eric: I’ve considered this. My concern is that, quickly, redtape begins to overlap with other gems such as Inherited Resources.

I didn’t document it but validations can be placed in the form. However, I’d strenuously recommend against using them for anything but fields that do not map directly onto a model’s field. Otherwise, the form validation begins to overlap responsibilities with the model’s validations.

Good thoughts, though. I’ve barked up similar trees already.