Check out this quote:
"The critical issue for Google is to segment commercial Gmail traffic from consumer traffic. Google must prove that consumer and commercial services are largely independent of each other, and that the commercial services will sustain at least a 99.9 percent uptime, which is the standard for most commercial e-mail SaaS (software-as-a-service) services."
Matthew Cain, Gartner, InfoWorld, "Update: Gmail Suffers Widespread Outage," September 01, 2009
For those who havent seen the news on this outage:
"Google Inc's efforts to persuade businesses to buy its email services
suffered a setback this week after a highly publicized outage shut down
accounts of millions of corporate customers for almost two hours."