We just discovered a hidden feature in Chrome which allows folder upload. As far as I know, this isn't supported in any other browser as of yet, but is supported in Chrome 7+.
Folder selection with SlickUpload 6
The new SlickUpload 6 RC2 has full support for folder selection and upload in Chrome. As additional browsers implement support, we will add support for them to SlickUpload.
Check out the updated SlickUpload 6 demo page or download the SlickUpload 6 RC2 package.