my slightly modified version of http://lincolnloop.com/blog/2009/sep/22/easy-fabric-deployment-part-1-gitmercurial-and-ssh/

sshagent_run.py

def sshagent_run(cmd):
    """
    Helper function.
    Runs a command with SSH agent forwarding enabled.
    Note:: Fabric (and paramiko) can't forward your SSH agent. 
    This helper uses your system's ssh to do so.
    """
    host = api.env.host_string
    result = api.local('ssh -A %s "%s"' % (host, cmd))
    print result