With more services existing “in the cloud”, reliability has become a chief concern, particularly for companies who offload existing services. Companies like Microsoft and Google who aim to be pioneers in web-based application services have an uphill battle in convincing people that they can be truly reliable. This year alone, for instance, there have been numerous outages for Google, including issues with Gmail, Google Docs and other services. Even if they are up “usually,” a business paying for a service wants more than that.
Google is now seeking to put their minds at ease with reliability promises, which most people refer to as service level agreements, promising a 99.9% uptime. That's a bit off of the “99.999%” uptime that some big hosting companies provide, but often even those claims are exaggerated. Google is promising the 99.9% figure with compensation available for paying customers who don't quite get it. So, if a paying business suffers an outage for too long, Google will compensate them.
As with many SLAs, Google would probably compensate them for the amount of “service” lost, not the amount of “money” lost – so a few hours of downtime would net them a few hours of service compensation, but not any money from lost sales or others. That could still be a problem for them in convincing larger businesses to switch to cloud computing, but they certainly are trying.