I'm running a wordpress 3.0.1. installation, where I inegrated uploadify for user contributions. After installing the goolge analyticor plugin http://wordpress.org/extend/plugins/google-analyticator/, the progress bar won't show up any more, and the upload doesn't really start.
If I disable the plugin, which actually integrates the asynchronous tracking code, it works again. Is there any know issue with uploadify and google analytics tracking scripts?
I don't know if i can help, but i had a similar issue. My webhost prints google analytics code under each page. So scripts like check.php etc may be returning some garbage code, instead of true / false or whatever.
My solution for this kind of issue is adding a die(); line at the bottom of the scripts (only if they are being called by ajax.