By my two-second calculations, if about half of the worlds population were using Bitcoin and making 2 transactions per day, a flat average would be about 70,000 transactions per second.  To account for peaks which one typically finds in network service use, better bump that up to about 250,000 transactions/sec.  Then one must consider the potential for DOS attacks.

Contrast this with the .25  (aka 1/4) transactions/sec at the current high.

As currently implemented, Bitcoin scales until it fails then that's all she wrote.  Ultimately I would be more comfortable with a solution which anticipated 'sharding' of some sort, and it seems logical and workable to combine this with a multitude of crypto-currencies each tuned to offer various advantages to their user base.

Of course the 'money changers' would have a hey-day in such an environment, but that fine.  Probably open systems would develop to make things less costly for end-users.  Some guy had a link to such a project a while ago.  I think it was on Docmeister's Bitcoin-2 post.