I have been having some trouble getting this to work as described in IE8. In other words, have the user be warned about the lack of flash and then roll on without resizing.
This might be a theme issue but I figured out that the following redirect:
location.href = location.href + "&you_toldmeabout_flash=donttellmeagain"
could be changed to:
location.href = "<?php echo add_query_arg('you_toldmeabout_flash','donttellmeagain');?>";
Otherwise the visitor ends up being redirected to a 404 not found error when they try to add an image from the Media -> Add New page.
It seems that the Media->Add New page does not have any arguments so the URL Arguments (_GET) should be prepended with a "?" mark in this instance. I just thought it would be more, WordPressy to use a wp function here.