I've got 500mb a month with my new android phone (Xperia 10) and I was just wondering if I download an app when I'm on a Wi Fi connection, will it take up my 500mb?
yes, I'm almost certain it does. But it depends on your provider. For Verizon, most data packages are unlimited use so you wouldn't have anything to worry about. But you can't be certain without double checking with your provider.
No it should not take up your data. Too make sure your on wifi, not 3g look at your notification bar on top. It will only show one or the other, so if it shows wifi then you will be using wifi, not your data. Hope this helps.