AFAIK you need Quantum (now named Neutron).
Have a look at Common Load-Balancer-as-a-Service Workflow.

But take care, there is a note one step before that:

The Load-Balancer-as-a-Service API is an experimental API meant to give early adopters and vendors a chance to build implementations against. The reference implementation should probably not be run in production environments.

UPDATE, April 2014: The above information seems to have been moved.
Link: http://docs.openstack.org/admin-guide...