The paid version of Google Apps includes a service level agreement that guarantees that the applications will be up 99.9% of the time. It works like this: If you are down for more than 0.01% of the time, they will add some days to the end of your contract–if you ask for it. The maximum amount you can get in a month is 15 days for 5% downtime.
5% downtime translates into about 36 hours. So if the service is down for more than 1.5 days each month, you’ll get an extra half a month. If the service is down for that long ever month and you ask for the credit, your contract will last 50% longer.
So basically if you are down for 100% of the time, you’ll only get an extra 15 days of service. Of course they are unlikely to be down for that long, but it does point out that the SLA really isn’t something that costs them anything to offer. That doesn’t mean it is a bad idea. The guarantee helps establish expectations and so far Google seems to do a reasonable job of meeting those expectations.