This is a really cool project that I found out through Jonathan Boutelle’s presentation at SXSW’08 called AJAX and Flash mistakes. That’s basically what they use on Slideshare to upload documents to their system.
It seems pretty useful, and I’m already thinking about using it in a few projects in the future. You can find more details about it on the homepage.
My only concern is that in some scenarios you may want to avoid a hard dependency on Flash, but for consumer sites it seems like a safe choice.