Terrific news, Amazon’s Simple Storage Service now has an SLA. This is one of the big features that they were missing and I suspect it’s going to go a long way toward making S3 more attractive as a commodity storage service.
As with a lot of these things, there are a bunch of caveats. They’re only providing service credits against future usage, not refunds, so if something goes totally kablooey and you want to stop using the service, you can’t get your money back. Also, you have to monitor your own success/failure rate (I suspect that S3 developers will start devoting more attention to logging and reporting on their use of the service; we do a lot of this for Approver.com already).
Even if they flawlessly maintain their 99.9% uptime guarantee (or if no of their users ever applies for the credit), simply drawing a line in the sand and saying "this is our uptime guarantee" is a big deal.