I have just added SWFUpload library as default way of uploading files, if flash player is not installed old upload appears. SWUpload allowed me to add upload progress bar, canceling during upload and new file explorer with ability to select many files.
Happily I was able to implement this only in JS so no PHP has been changed.
All code is in r39654(jsparty) and r39365(cms).
SWFUPload homepage: http://swfupload.mammon.se
Our version comes from: http://linebyline.blogspot.com/2007/07/swfupload-revision-6-release.html
IÃ¢â‚¬â„¢ve recorded short demo: http://www.ii.uj.edu.pl/~ujma/upload/upload.htm
You can check it also here http://test.silverstripe.com/mujma (admin/password)
I would love to get feedback about it.