I can't help but be a little surprised that Heroku's entire build system is disabled by an S3 failure in one region. Now I'm unable to add a notice about the issues to my site's HTML...

True. Given the business Heroku is in, they should have redundancies in multiple regions if not multiple service providers.

Since S3 is the defacto artifact delivery system for most people that run on AWS, it's not much of a surprise. For the most part, very isolated incidents aside, S3 is rock solid. Even EC2 relies on S3 for launching non-EBS instances.

Heroku build logs spitted this out:

    Unable to fetch source from: https://s3-external-1.amazonaws.com/heroku-sources-production/heroku.com/<some-uuid>?AWSAccessKeyId=<some-access-key>&Signature=<some-signature>&Expires=1439198046