That is a good question and I am sure it's been discussed before because it's a logical one. I have done it over 3G and my Wifi. I've never done it over some unsecured WIFI though. I'd say on my secure WIFI, yeah. Over 3G, I'm going to be honest and say I don't know. I haven't bought a movie yet (I have that app for that purpose) but I have checked in for flights, shopped and bought things on eBay (on a plane while waiting to take off), and looked at my bank accounts.
I don't have an answer, sorry, but I think it's a good question.