I had always assumed that if the wifi icon was showing it was using data through wifi, if the 3g icon was showing it was using cell data. I have unlimited so never really cared to find out if that was the case. I would assume that is the case though.
so technically, with a dummy SIM card, I can use my windows phone as a media player then right? Netflix, youtube, and the zune cover arts/artist bios will all work without a data plan?