Haven’t tried it, but it is possible that Pound is routing the successive requests from each user back to the same mongrel process. This is important because each mongrel only knows about it’s own set of uploads unless you are using the DRb version of mongrel_upload_progress to synchronize all of your mongrels. I’m using the uploader behind Pen with the DRb synchro version and it works just fine. Pen is capable of trying to remember which user belongs to which backend server, but I’m not using that functionality and am opting for DRb to do the sync instead.
Check out the DRb section on: http://mongrel.rubyforge.org/docs/upload_progress.html
Hope this helps…