I tethered my wifi-only iPad to my iPhone using Mywi and did some very light internet browsing for roughly 60-80 minutes, no streaming video or music. Watching the up/down data displayed the iPhone's "usage" section, I found that the amount of data used was much more than similar browsing would be on the iPhone alone. I usually use approximately 10MB a day on my iPhone, and yesterday I used just over 50MB in that relatively short surfing session. As a test, I opened the extremely nice Weatherbug app and measured the amount of down data used just to open the app and fully display all the info for my default weather location, with no additional use inside the app. Each time I tried used over 1 MB of data. I had been considering the iPad 3G, because I thought the 250 MB data plan would be plenty... After all, I usually don't break the 300 MB mark on my iPhone in a month. However, if these numbers are accurate, there's no way 250 MB will be sufficient for even light usage and the $30 unlimited plan would be required. YMMV.