On mental models of persistence

Sara Allen posted a thoughtful article on persistence in web-applications/RIA.

Rethinking persistence in desktop applications seems to be a worthwhile endeavour, too. With todays 100GB+ hard-drives, multiple GHZ-machines and multiple-undo system architectures, why make it a requirement for the user to explicitly save a document? Why not saving it incrementally? Thus bringing versioning into the picture for the average user?

On a side note, explicit saves ("commits") are considered standard in database-applications, however there a database system which supports automatic persistence: FileMaker. To my knowledge, every keystroke/change to a record/table field/columns is made persistent automatically.