I've mentioned something along these lines before as well. I want it to be seamlessly integrated with Aperture (and likely Lightroom, given how things are going in Aperture-land) such that I can browse all of my photos at any time. The RAWs are securely backed up in some datacenter, and if I want to edit the photo, the RAW is cached locally so that it can be edited. If the photo hasn't been edited in some time now, it's evicted from the cache, and I still have the preview available for perusal.