Some interesting findings from web-dev land…
I was experiencing this issue with some larger file uploads using the CakePHP Media plugin.
https://bugs.webkit.org/show_bug.cgi?id=5760
Add this to the virtual host declaration in your Apache config file to fix it:
123456<VirtualHost x.x.x.x:x>
# Prevent Safari from hanging on uploads
BrowserMatch Safari nokeepalive
</VirtualHost>