I am playing with plupload 2.1.3 and can't get it to complete an upload using their custom example. I get a 103, unable to move uploaded file. If I look in the upload directory I see the file I tried to upload listed as test.png.part. My php.ini lists /tmp as my upload_tmp_dir.
From this I gather that the permissions are correct (I even set everything to 777 temporarily -- go difference).
This is a fairly newly configured server so it may not be related to plupload.