If you own your own IP's and have geographically diverse data centers spread over a dozen or more major carriers, its not technically impossible, but one bad BGP4 announce could ruin your whole day ( and SLA )
But even if that happened, most site monitors would see a 404 and many would report it as still 'up'
100% is a marketing gimmick, its more like 99.99999
But, when properly engineered, giving a SLA credit is rare
Our Xen cloud spans across 5 data centers in the US, plus several in Europe, all using distributed shared storage. Its very unlikely that you'll even notice the ~10 seconds it takes for your VM to show up with the same IP in another data center if a problem happens.