Internet Explorer has a file download limit of 4GB (2 GB on IE6). Firefox does not have this problem (haven\'t tested safari yet)
(More info here: http://support.microsoft.com/
Blizzard use a light-weight .exe BitTorrent wrapper for their patches. I'm not entirely sure how it is done, but it looks like a branded version of the official BitTorrent client.
Upload to Amazon S3, provide the torrent link of the file (all S3 files are automatically BitTorrent-enabled), plus the full HTTP download link as alternative. See S3 documentation