Technically, Plone supports many means of mass-uploading content, esp files / images:

• FTP
• WebDAV, also Enfold Desktop
• PloneJUpload

Unfortunately:

• WebDAV relies on an awkward implementation which also breaks too often
• Some WebDAV clients do things which do not make sense and are difficult for us to workaround
• JUpload seems to be broken in JRE 1.5, and PloneJUpload seems also to be broken

Our community definitely needs answers to this question, which may include:

• Contributing directly to JUpload
• improving WebDAV support on our end and being vocal with developers of client software and/or contributing directly
• Developing new approaches, perhaps in the style of the GMail attachment widget or Flickr uploader app for Win, OSX, Linux, etc..

Look at the z3c.widget product which provides a Flash multi-file upload widget in Zope 3. Try to make this work in Plone.