That's interesting.
Most good (or "non creepy") CDN providers will have a TTL on cache files, and also not copy things unless you have asked them to!
And the only way to see if they have a copy of something is to ping a URL, which causes them to make a copy. That is why the DWaves blog thought they were being given a list of URLs as soon as they were uploaded to a blog, because as soon as they were uploaded to the blog they could be found at i0.wp.com - i1.wp.com and i2.wp.com its a catch-22.
By indefinitely I mean that its their discretion how long files live on their CDN, and outsiders don't know the policy.
Edit: someone has checked, and they do not respect robots.txt A domain can reject all crawlers and Automattic will still copy files from it for anyone who pings their domains.