No it's not a routing protocol, but it makes the router so configurable that you're able to switch the protocols out easily. You could even invent your own for your internal network. I guess this is what Google has done because they have so much knowledge about their traffic and infrastructure. It's not only about routing protocols though. It also allows you to do traffic shaping and all kinds of cool stuff as well. Just imagine a software router which is just as high speed as one which implements the logic in hardware.

My impression is that OpenFlow is most useful for internal networks. Instead of having smart switch, you have controllable switches and a smart controller. But I guess it would be useful for splitting up border routers into a switch and controller that does the routing and talks BGP.

Think of it as an automated method for providing configuration convergence across an infrastructure, instead of relying on manual input of routing tables.