From the devguide:

"If users attempt to upload more that 5GB with this method, the server will close the TCP/IP connection after 5GB and purge the customer data from the system. Users must take responsibility to ensure the data they transfer will be less than 5GB or to split it into 5GB chunks each in its own storage Object."

Make the client preemptively throw an error if the file is to large.

Don't know if this is implemented or not on this implementation, but I also opened http://github.com/rackspace/ruby-cloudfiles/issues/issue/10