this seems to be a larger issue with iOS. Do a quick google for "force iphone to use wifi" and you'll see what we mean. From scanning some of these posts, it seems the sure way to at least know which it is using is by verifying that you see wifi bars in your menu bar. Sometimes (apparently) the wifi connection does not immediately establish after waking and if a network activity is requested, it may use the data plan instead, finishing that network request with cellular data even if wifi subsequently becomes available during the transfer.

To be honest, we don't even known how the dropcopy transfer was even possible over 3G since that is presumably a completely different network segment than your wifi network – but network work in mysterious ways sometimes…