I was tinkering with Quora and posted a link to see what they used for validation and sure enough they're using AWS. Finally, something useful that gives me an actual reason to punch a hole in the AWS firewall.
Not sure why they needed to check it 4 times. Two IPs properly identified their user agents, the next two used default python UAs, pretty sloppy programming all around.
Sadly, punching a hole in the firewall for them leaves a gaping hole for scrapers using AWS. Need to put my noodle to work and figure out some ID scheme that allows vendors to use a shared modem pool and ID themselves without using rDNS because this situation is only going to get bigger as more sites transition to cloud computing.