The 100MB chunks may help.
The fact that individual connections may still vary (depending on ISP) is another argument for hosting in the UK. Most major ISPs peer at LINX in London (and other similar facilities) this helps keep traffic inside the UK and speeds up data transfer. So whilst there may be no underlying problem with box.net or the transatlantic bandwidth in general, it doesn’t remove the need to host the files in the best possible way for end users (even if any problems are not directly related to the infrastructure we choose to use). Anyway, it is more environmentally responsible to host nearest to your target audience