We've talked about these kinds of scaling issues in the past, and we've got a ways to go until they become an issue.
I still strongly believe that we shouldn't worry about them until they actually start to become a problem.
Long before we get to 300 million transactions per day there will be big companies spending lots of engineering dollars on their own highly optimized versions of bitcoin.
I bet there will be alternative, secure-and-trusted, very-high-speed network connections between major bitcoin transaction processors. Maybe it will just be bitcoin transactions flying across the existing Visa/MasterCard/etc networks (I have no idea what their transaction clearing/processing networks look like or how they work).
Instead of worrying about "what will we do when we are wildly successful" we should be worrying about "what should we do RIGHT NOW so we become wildly successful."