Five '9's not really appropriate for rented web servers.
The 2,3,4,and 5 '9's is a baseline SLA tool designed to show how much downtime a system can tolerate before it is considered to affect the purpose of the system. For example in the case of a banking system, that is how long before the cost of bringing the system up is lower than the money you are losing, which in most cases is a very very short time indeed, so we aim for the 5 nines.
In the case of rented hosting, it's probably reasonable to suggest that a longer period of downtime should elapse before compensation payments are triggered.
The downtime is worked out roughly like this:
One nine 90.0% 36 days 12 hours
Two nines 99.0% 87 hours 36 minutes
Three nines 99.9% 8 hours 46 minutes
Four nines 99.99% 52 minutes 33 seconds
Five nines 99.999% 5 minutes 35 seconds
Six nines 99.9999% 31.5 seconds
So, over a year, 99% means that about 3.65 days downtime is expected. A hosting company that offers you an SLA of 99% is saying that you can't really complain until the overall downtime in the last 12 months reaches 3.65 days.
A 99.9% SLA is 8 hours in a year and so on.
Five '9's is only 5 minutes or so. I think it would be unreasonable to ask for a low-cost shared hosting company to aim for a five '9's target, so we should probably be sceptical of those that claim they can.
Uniquely, Rackspace offer a 100% Infrastructure Availability Guarentee. Which suggests they pay out for anytime your site or server is not available. That definitely sounds fanatical, and I actually find that promise more believable that some company that throws in the term 99.999% uptime.
However I am not a Rackspace customer, but I would be interested to know if any of their customers received credits or cool hard cash as a result of todays power outage.