Here's a patch for a new image uploader. The visual design is to spec for the new UI. It's one portion of the new media library popup for the post edit page.
Uploading and sending to the editor happens in a single step: clicking the Add Image button does the upload, inserts the attachment post, sends the img src html to the editor, and closes the popup.
The popup is handled by Thickbox, with the contents in an iframe. I made some wp-specific changes to the Thickbox js: $() changed to jQuery(); addLoadEvent() instead of read() (thanks mdawaffe); and the close link changed to a [x] png as per the design spec.
I've attempted to split the code into functions that are specific to the image uploader (functions named image_*()) and functions that will be useful for other media uploaders (video, audio) - though they'll probably need some tweaking. There's a trivial hook for other media uploaders in the form of the media_upload_{$type} action in wp-admin/media-upload.php.
Also yet to be implemented are the other tabs on the image uploader ("Media Library" and "Flickr"). They're just hard-coded for now. I considered a flexible tab system like the old $wp_upload_tabs thing, but I think it'd be better to go with something simpler instead, like a simple action.
All the old uploader code is still in place for now, but should be removed once this is solid enough.
It would be possible to do this using Ajax instead of an iframe but the iframe approach seems to offer a better fallback and more reusable code.
