Boto randomly connecting to different regions for S3 transfers
The problem ended up being an internal billing error at AWS and was not related to either S3 or Boto.
I have several S3 buckets containing a total of 40 TB of data across 761 million objects.  I undertook a project to copy these objects to EBS storage.  To my knowledge, all buckets were created in us-east-1.  I know for certain that all of the EC2 instances used for the export to EBS were within us-east-1.
The problem is that the AWS bill for last month included a pretty hefty charge for inter-regional data transfer.  I'd like to know how this is possible?
The transfer used a pretty simple Python script with Boto to connect to S3 and download the contents of each object.  I suspect that the fact that the bucket names were composed of uppercase letters might have been a contributing factor (I had to specify OrdinaryCallingFormat()), but I don't know this for sure.