This appears to be resolved. AWS posted an update to their status page, which matches some of the observations in this thread about it being ISP-specific:
[RESOLVED] External Connectivity Issues
Between 11:43 AM and 1:30 PM PST some customers may have experienced connectivity issues between an external provider outside of our network and AWS destinations. This issue impacted connectivity originating from California. Connectivity to instances and services within the Region was not impacted during this issue. All AWS Services have been, and continue to operate normally.
[RESOLVED] External Connectivity Issues
Between 11:43 AM and 1:30 PM PST some customers may have experienced connectivity issues between an external provider outside of our network and AWS destinations. This issue impacted connectivity originating from California. Connectivity to instances and services within the Region was not impacted during this issue. All AWS Services have been, and continue to operate normally.