Here's [1] their official SLA. This outage so far brings them to less than 3 nines of uptime this month (43.8 minutes) but still more than 2 nines (7.2 hours) so it sounds like everyone gets 10% off their S3 bill.

Very curious if Amazon will apply this automatically or only if you complain.

Edit: from further down the same page, it looks like only if you write in to support do you get these broken SLA credits. Kind of lame since everything else about their billing is so precise and automatic.

[1] https://aws.amazon.com/s3/sla/