So 100% uptime is really difficult to achieve, hardware wise. Software wise you'll have to prove that there are no bugs in the system that might bring it down. That is much, much, harder.
You can have 100 servers in 100 different countries and have the client automatically change to another server if the one they are connected to goes down. But if there is a software bug that crashes all your clients on start-up, or worse, crashes all your servers (think what happened to Skype not long ago).
Also, never underestimate bugs in hardware (pentium 1). You'll need multiple locations, multiple hardware, multiple operating systems, multiple compilers, multiple versions of the software.... Standardizing on one of these components may bring down your entire system!
You can have 100 servers in 100 different countries and have the client automatically change to another server if the one they are connected to goes down. But if there is a software bug that crashes all your clients on start-up, or worse, crashes all your servers (think what happened to Skype not long ago).
Also, never underestimate bugs in hardware (pentium 1). You'll need multiple locations, multiple hardware, multiple operating systems, multiple compilers, multiple versions of the software.... Standardizing on one of these components may bring down your entire system!