Will Amazon or Rackspace be hosting both your private and public cloud? If so, this would be a question for them. If you are personally managing a part of the cloud then you need to ensure that whatever distributed system architecture you're using to distribute workload uses secure means to transfer jobs/data/tasks. I doubt any modern distrib system architecture would send data over insecure channels, though.
In the off chance that the architecture does not support secure distribution of data, a VPN would be an easy and simple solution, though.