The basic gist of it is... Often when I tether my Macbook while traveling it'll start download large amounts of data from iCloud. Photostream is a big culprit, to the point that I just turn it off. iMessage too, as it syncs conversations that are often full of images. Is there a hidden switch somewhere to NOT download bulk data when tethered? The machine KNOWS it's tethered; it uses a different WiFi icon (the chain links) when tethered. So this would be trivial to implement. The worst case was a few months ago, when it blew through nearly a gig of my data plan before I realized it was downloading hundreds of photos I had taken on my phone on my trip. Ugh.