Thomas Larsson has posted this cool little tutorial today that gaves a method (without patching PHP) to allow for multiple asynchronous file uploads.
One of the few things that I find lacking in PHP is the ability to report the progress of a file upload. This means that file uploads, especially uploads of larger files, can be extremely frustrating for end users when they don't know if the upload is progressing or if it has stalled or if it has even started.
He mentions another way around it (a patch from Pdoru), but that requires the previously mentioned patching of the install. His method actually uses a seperate script (called via an iframe) to make the upload while still leaving the page able to be used. The Prototype library is used to update the progress bars based on the amount of the file that's been uploaded (as reported by a PHP script).