There is a bug in either swfupload or Flash itself that sometimes causes the server response from a file upload to be truncated in Linux and Mac OS:
http://code.google.com/p/swfupload/issues/detail?id=4&colspec=ID%20Type%20Status%20Summary%20FixReleasedIn%20FixTargetedFor
It appears only to happen on certain servers. I've been able to reproduce this using my MacBook? - it fails on some WP installs but not others. I haven't been able to determine the cause (nor have the swfupload developers it appears) but I suspect it's a bug in the Flash Player's TCP/IP code.
The enclosed patch works around the problem by greatly reducing the size of the server response to a Flash file upload. Instead of returning a big chunk o html on success it returns the ID of the uploaded file. The html detail form for the file is then fetched in a second ajax request.