There is a lot of rumors that Google already uses SDN inside its data centers as well as to route and control the external traffic between them.
According to this article
Going With the Flow: Google’s Secret Switch to the Next Wave of Networking
Google is operating an SDN enabled network. But can you really believe in everything what you read? Researching more on the the topic of SDN plus Google I have found this more skeptical article. It says that this may not be a whole truth Openflow @ Google: Brilliant, but not revolutionary
Despite this to contradicting articles It would be nice to know that SDN can scale and has already proved its usefulness on a large scale. But without this there is still the open question you have to ask your self.
Is SDN really going to transform and shape the traffic within and outside of small or big data centers. Will it really provide a more sophisticated ways to control and direct the external traffic over wan links as well as over the 10 GiB enabled internal networks? Without any prove of concept (POC) this all is only one idea more that may or may not get bigger traction in networking.
To finis this up I would like to refer refer to this article Balloons, Bags, and SDNs . There is one very useful and I particularly smart conclusion about the whole SDN future:
Before you say, “it won’t scale,” ask, “compared to what?”
With it in mind let's wait how the market is going to move and what expectation will or will not be fulfilled :).