@mrdavidlaing - S3 has BitTorrent  support build in, see [Using BitTorrent with Amazon S3](http://docs.aws.amazon.com/AmazonS3/latest/dev/S3Torrent.html):

All you need to do is add `?torrent` to any file, e.g. for the box at hand it would be `https://s3-eu-west-1.amazonaws.com/ci-vagrantboxes/pressupbox-development-boilerplate-20130327.box?torrent`.

I've always wanted to add support for this to Vagrant itself (or suggest it at least for starters), but haven't gotten around to it so far.