Good thought, but it tends not to survive contact with the real world.

There were many attempts at this early in the evolution of XML Schemas. 
None of them were really particularly usable/useful; schemas don't 
generally contain enough metadata about the meaning of the information 
to let you generate a really user-friendly set of forms, and so the idea 
didn't really catch on. You wind up with a much better user experience 
if you take the time to hand-implement the interaction.

Also, forms entry doesn't deal very well with optional sections and 
recursion -- you'd need to spawn sub-forms based on interaction with the 
user, which doesn't fit neatly into the concept of a single HTML form. 
(I'm not sure whether XForm does any better in that regard.)

There are certainly schema-directed XML editors, but they tend to be 
aimed at folks who are already near-experts in the specific schema 
they're working with and hence don't need a lot of user prompting -- or 
rely on schemas having been annotated with additional information to 
guide the user interface generation. And they rely on presenting a 
richer set of interaction techniques than a simple form can provide.

-- 
() ASCII Ribbon Campaign  | Joe Kesselman
/\ Stamp out HTML e-mail! | System architexture and kinetic poetry