In the last few days I have seen some really fast downloads. Gigabytes of files transferred between providers over ssh within minutes, and some very fast http downloads. In the SSH transfer it felt as though they must have been in the same DC.
For instances I downloaded VirtualBox to a server and it complexed in seconds. If felt as though some kind of system detected the file name, realized that it was available locally and simply added some kind of symlink for it rather than download the whole thing. it didn't feel like it was cached.
In the case of the http there something going involving transparent caching at ISPs I should know about?