Doing Zero Downtime Deployments with Bluemix is important for developers. Although it is not yet supported by Bluemix, support is coming in the next release of the Bluemix Droplet Execution Agent "DEA".
However it can actually be implemented already, as Jeff Sloyer describes in his very good blogpost Zero Downtime Deployments with Bluemix. More information on this important topic can be found on this link: A script is available to assist in blue-green deployments and Zero Downtime Deployments Using IBM Bluemix. Any questions let us know.