Amazon's S3 getting more interesting
The new SLA includes in my opinion exactly such a statement with a 10% credit should the monthly uptime percentage fall below 99.9% (and even 25% for less than 99% uptime). This agreement won't be really all that interesting to private customers, though, since it requires that you yourself protocol when according to you S3 had an outage and notify Amazon about this issue.
But S3 thanks to its architecture is more suited for professional in the first place, anyway.
[via Fred Oliveira]